bitbake: cooker: Rename confusing 'stop' state to 'forceshutdown'

bitbake: cooker: Rename confusing 'stop' state to 'forceshutdown'

The shutdown state causes the server to finish what its doing, stop was
them meant to completely stop it. It doesn't mean the server is stopped
though. Renaming the current stop event for forceshutdown gives more
meaning to what it actually does. The stopped namespace then becomes
available to indicate a completely stopped server.

(Bitbake rev: 12e9d33bfae5294e3870dfd1202f63383ad05e92)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>