skip deleting of persistent volumes by default (#723)

This commit, intruduces changes to the current behavior of the "make cleandocker" command.
From now on, an extra parameter must be given in order to delete the persistent volumes,  which up until now was done by default. The new command which represents the old behavior (deleting all artifacts created from the docker composition) is "make pv=true cleandocker".

Signed-off-by: Jovan Kostovski <chombium@gmail.com>

Makefile

@@ -45,7 +45,7 @@ cleandocker:
 	# Remove old mainflux images
 	docker images -q mainflux\/* | xargs -r docker rmi
 
-ifndef spv
+ifdef pv
 	# Remove unused volumes
 	docker volume ls -f name=mainflux -f dangling=true -q | xargs -r docker volume rm
 endif

docs/dev-guide.md

@@ -65,7 +65,7 @@ make docker_http
 
 > N.B. Mainflux creates `FROM scratch` docker containers which are compact and small in size.
 
-> N.B. The `things-db` and `users-db` containers are built from a vanilla PostgreSQL docker image downloaded from docker hub which does not persist the data when these containers are rebuilt. Thus, __rebuilding of all docker containers with `make dockers` or rebuilding the `things-db` and `users-db` containers separately with `make docker_things-db` and `make docker_users-db` respectively, will cause data loss. All your users, things, channels and connections between them will be lost!__ As we use this setup only for development, we don't guarantee any permanent data persistence. If you want to update your Mainflux dockerized installation and want to keep your data, use `make spv=true cleandocker` to clean the containers and images and keep the data and then `make run` to update the images and the containers. Check the [Cleaning up your dockerized Mainflux setup](#cleaning-up-your-dockerized-mainflux-setup) section for details. Please note that this kind of updating might not work if there are database changes.
+> N.B. The `things-db` and `users-db` containers are built from a vanilla PostgreSQL docker image downloaded from docker hub which does not persist the data when these containers are rebuilt. Thus, __rebuilding of all docker containers with `make dockers` or rebuilding the `things-db` and `users-db` containers separately with `make docker_things-db` and `make docker_users-db` respectively, will cause data loss. All your users, things, channels and connections between them will be lost!__ As we use this setup only for development, we don't guarantee any permanent data persistence. Though, in order to enable data retention, we have configured persistent volumes for each container that stores some data. If you want to update your Mainflux dockerized installation and want to keep your data, use `make cleandocker` to clean the containers and images and keep the data (stored in docker persistent volumes) and then `make run` to update the images and the containers. Check the [Cleaning up your dockerized Mainflux setup](#cleaning-up-your-dockerized-mainflux-setup) section for details. Please note that this kind of updating might not work if there are database changes.
 
 #### Building Docker images for development
 
@@ -111,7 +111,7 @@ __Note:__ Please store your customizations to some folder outside the Mainflux's
 
 
 ### Cleaning up your dockerized Mainflux setup
-If you want to clean your whole dockerized Mainflux installation you can use the `make cleandocker` command. Please note that __by default the `make cleandocker` command will stop and delete all of the containers, images and persistent volumes__. If you want to keep gathered data in the system (the persistent volumes) please use the following command `make spv=true cleandocker` (spv = skip persistent volumes). This form of the command will stop and delete the containers and images, but won't delete the persistent volumes.
+If you want to clean your whole dockerized Mainflux installation you can use the `make pv=true cleandocker` command. Please note that __by default the `make cleandocker` command will stop and delete all of the containers and images, but NOT DELETE persistent volumes__. If you want to delete the gathered data in the system (the persistent volumes) please use the following command `make pv=true cleandocker` (pv = persistent volumes). This form of the command will stop and delete the containers, the images and will also delete the persistent volumes.
 
 
 ### MQTT Microservice