Update Alembic database scripts for external scripting and PostgreSQL, Oracle

This patch does the following:
1) The env scripts have been updated to be tolerant of a NULL configuration
   file. This occurs when configuration is provided by an external script,
   such that the actual config.ini file is not used.
2) Enum types have all been given names. This is needed for PostgreSQL script
   generation.
3) The identifier meetme_confno_starttime_endtime is greater than 30
   characters, and hence invalid for Oracle databases. This has been truncated
   down to meetme_confno_start_end.
........

Merged revisions 400383 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@402073 65c4cc65-6c06-0410-ace0-fbb531ad65f3