Remove disable_service from DBs configuration

Remove disable_service from DBs configuration

Remove disable_service from configure_for_func_testing.
A recent Devstack patch (Linked in bug report) checks
that a disabled service is not enabled later. This breaks
the code this patch touches. I believe the DBs were disabled
and enabled with the assumption that Devstack expects only
a single DB to be configured at a time, but that doesn't
seem to be the case. Simply removing the disable calls seems
to work fine.

Also exclude oslo.messaging==2.6.0 as per global-requirements.txt.

Closes-Bug: #1505259
Closes-Bug: #1505295

Depends-On: I47ab12f719fba41c2f0c03047b05eb28f4423682
Change-Id: I0fbcc1218e1aa9adbf89f6121c1c2e224ed93cab