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

contrib/ast-db-manage/config/versions/189a235b3fd7_add_keep_alive_interval.py

@@ -0,0 +1,22 @@
+"""add_keep_alive_interval
+
+Revision ID: 189a235b3fd7
+Revises: 28ce1e718f05
+Create Date: 2015-12-16 11:23:16.994523
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '189a235b3fd7'
+down_revision = '339a3bdf53fc'
+
+from alembic import op
+import sqlalchemy as sa
+
+
+def upgrade():
+    op.add_column('ps_globals', sa.Column('keep_alive_interval', sa.Integer))
+
+
+def downgrade():
+    op.drop_column('ps_globals', 'keep_alive_interval')