Alembic: Add PJSIP global keep_alive_interval.

The keep_alive_interval option was added about a year ago, but no
alembic revision was created to add the appropriate column to the
database.

This commit fixes the problem and adds the column. This was discovered
by running the testsuite with automatic conversion to realtime enabled.

Change-Id: If3ef92a7c4f4844d08f8aae170d2178aec5c4c1a