Fix alembic script to work properly in offline mode.

When run in offline mode, this would attempt to check the database for
the presence of a type it was going to try to create. I now check the
context to see if we're running in offline mode and change a parameter
accordingly.
........

Merged revisions 407567 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@420237 f38db490-d61c-443f-a65b-d21fe96a405b