exclude the mysql_init (localhost) from the common roles

[thraxil]

Fixes: https://appsembler.atlassian.net/browse/OT-480

We want these roles applied to all hosts, but the Tahoe inventory files have special entries like:

[mysql_init]
127.0.0.1

Targetting all causes it to want to run those on 127.0.0.1 as well, which clearly doesn't work.

We could either directly include all the other groups with something like:

hosts: edx,services,workers,forum

But then if we someday add another group (eg, if we merge the Metabase setup into this playbook), we'd need to remember to include it there as well.

Instead, we stick with all, but then explicitly exclude the one odd one.

(a separate question is whether we even use that mysql_init group for anything or if it's leftover from previous work)