Support rerunning the installation script by restarting postgresql.

If postgresql was installed and started in a previous run, start causes:
Error: Service `postgresql` already started, use `brew services restart postgresql`

When developing changes to these scripts, it's important to be able to run and rerun this script without removing applications.
Merge pull request #235 from ManageIQ/support_reruns_where_pg_is_started

Support rerunning the installation script by restarting postgresql.