LBaaS: synchronize haproxy deploy/undeploy_instance methods

LBaaS: synchronize haproxy deploy/undeploy_instance methods

When creating/deleting lbaas objects at a high rate (like tempest api tests do)
deploy/undeploy_unstance methods of haproxy namespace driver
may be executed concurrently which leads to different conflicts with
network namespaces and interfaces created by the diver.

@synchronized decorator should be used in order to ensure that only one thread
will execute one of the methods at a time.

Closes-Bug: #1260692

Change-Id: Ibe86653c20c9a4e0b4e7b3b2cefbd445428b82da