[setup.sh] preserve GAZEBO_MASTER_URI if set

[moriarty]

If GAZEBO_MASTER_URI is set use that otherwise use the default.

Most of the environment variables set in setup.sh already preserve or append to values which were already set.

Example use case:
If you want to run gzserver and gzclient in two different containers. You would first start the gzserver and then pass the GAZEBO_MASTER_URI to the docker run -e for the gzclient which could now source setup.sh without needing it's own custom entrypoint.

The only environment variable in setup.sh which does not preserve or append is GAZEBO_MODEL_DATABASE_URI - I imagine it may be worth doing the same change to that variable too.

[moriarty]

wow that's a lot of ✖️ checks failing

[j-rivero]

@osrf-jenkins run tests please

[j-rivero]

all builds are UNSTABLE (expected at this moment) and test failures are not relevant to this PR as far as I can see.

I thought the change would bring some bash particular syntax to the file but seems like variable default values are in the posix standard, good.

[j-rivero]

Thanks @moriarty ! I final request, could you please update the Changelog.md file with an entry about this change?

[j-rivero]

Thanks Alex!