Remove SELECT FOR UPDATE use in ML2 tunnel driver add_endpoint

Remove SELECT FOR UPDATE use in ML2 tunnel driver add_endpoint

SELECT FOR UPDATE expression, which is triggered with the use of the
SQLAlchemy Query object's with_lockmode('update') method, is
detrimental to performance and scalability of the database
performance code in Neutron due to the lock contention it produces.

SELECT FOR UPDATE can be entirely avoided in add_endpoint methods
with the use of single-shot SELECT and INSERT expressions and the
correction of VxlanEndpoint primary key: indeed previously it was not
possible to create multiple endpoints with the same ip, now the model
primary key constraint ensures it.

Change-Id: Id69fbc15c8f51b4b275cd742312e6ff6802d8c0f
Partial-Bug: #1330562