pjsip realtime: already created enum failure for postgresql

If an enum had been previously created the alembic script would attempt to
re-create it and an error would be generated while running migrations for a
postgresql server.  The work around for this is to use the ENUM object type
for postgres as opposed to the generic enum type used by sqlalchemy. Using
this type in the script seems to work properly for both postgres and mysql.
........

Merged revisions 407572 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@407574 f38db490-d61c-443f-a65b-d21fe96a405b