Big Switch: Only update hash header on success

This patch moves the hash update call into the success block
of the server manager so the database isn't updated with
a hash header from an error response. Additionally, it prevents
the hash from being updated to an empty value if the hash header
is not present in the response.

Conflicts:

	neutron/tests/unit/bigswitch/test_servermanager.py

Closes-Bug: #1348766
Change-Id: I512d01f9bb91b208dd58883d2464951ecc6748e1
(cherry picked from commit b8cf45218714937681d7df2e5b9d7d440cb2edd9)
Merge "Big Switch: Only update hash header on success" into stable/icehouse