Add registry migration subcommand to Omnibus' gitlab-ctl

Context

As we move Allow manual implementation of container regist... (&10757) forward, we will need to expose a way for admins to execute schema migrations for the registry manually too as described in #1102 (comment 1533217349).

Omnibus already exposes a subcommand to execute offline garbage collection through registry-garbage-collect (see implementation).

Solution

Provide a subcommand to gitlab-ctl such as registry-database migrate that can execute migrations in the up or down direction. The command should be a reflection of the registry's bin/registry database migrate up/down that takes the same arguments.