Respect VERBOSE environment variable if already set before in verbose mode

[meyerj]

At the moment it is not possible to build a workspace with catkin_tools in verbose mode without also switching make to verbose mode.

For our continuous integration jobs I would like to have a way to see the normal output of cmake and make (with catkin build -v), but without all the clutter produced by make if the VERBOSE environment variable is set. The alternative is -i, but it does not behave well if the output is not a terminal and if multiple packages are built in parallel.

Note that setting VERBOSE=0 does not have the desired effect. The variable needs to be defined, but empty to disable make's verbosity (VERBOSE= catkin build -v ...).

Maybe the whole section was a leftover from debugging catkin_tools? The GitHub issue template explicitly asks to set VERBOSE=1 although -v already implies that in the current version:

[//]: # (Use `VERBOSE=1 catkin build -p1 -v` for verbose output)

[mikepurvis]

The CI fail is due to flakiness on OS X that's unrelated to this change. Thanks for the contribution!