Add Postgresql task type

[Sda79]

Add PostgreSQL tasks

SIPssert only supports MySQL database. I added PostgreSQL tasks in a very similar way to MySQL.
I am aware that it surely need improvement, at least documentation, please tell what to improve/fix.

As MySQL you can add a root password with postgres_password and every SQL files in the scenario directory will be loaded by Postgres at startup.

A simple example scenario could be :

---
timeout: 60

tasks:
 - name: PostgreSQL
   type: postgresql
   stop_timeout: 5
   ready:
     wait: 50

 - name: Opensips
   type: opensips
   require:
     ready: PostgreSQL

[razvancrainea]

This is great, thanks a lot for your contribution!