Fedora/RHEL DB updates for devstack_gate.sh

If you're using Fedora or Red Hat in your jenkins server,
It will always fail when check the database, because
on Fedora, the path for postgres and mysql directories
are different from Ubuntu.
This will check which distro are being used and check
the right path.

Change-Id: Ib1df1c9be92ce5a506caa5b36b69dfab32d9a99f
Merge "Fedora/RHEL DB updates for devstack_gate.sh"