res_pjsip: Add contact_user to endpoint

contact_user, when specified on an endpoint, will override the user
portion of the Contact header on outgoing requests.

Change-Id: Icd4ebfda2f2e44d3ac749d0b4066630e988407d4

contrib/ast-db-manage/config/versions/4e2493ef32e6_add_contact_user_to_endpoint.py

@@ -0,0 +1,22 @@
+"""Add contact_user to endpoint
+
+Revision ID: 4e2493ef32e6
+Revises: 3772f8f828da
+Create Date: 2016-08-16 14:19:58.918466
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '4e2493ef32e6'
+down_revision = '3772f8f828da'
+
+from alembic import op
+import sqlalchemy as sa
+
+
+def upgrade():
+    op.add_column('ps_endpoints', sa.Column('contact_user', sa.String(80)))
+
+
+def downgrade():
+    op.drop_column('ps_endpoints', 'contact_user')