update vsm credential correctly

Today if we modify the n1kv VSM credential in the cisco_plugins.ini, the
older VSM ip address remains in the db, and all requests are sent to
the older VSM. This patch deletes all n1kv VSM credentials on neutron
start up before adding the newer VSM credentials. Hence making sure
that there is only one n1kv VSM IP address and credential in the db.

Change-Id: I772a86bf896a1d3d9c69c545ce6918b0fe3a2e48
Closes-Bug: #1341014
Merge "update vsm credential correctly"