Big Switch: Only update hash header on success

Move the hash update call into the success block
of the of the server manager so the database isn't
updated with a hash header from an error response.

Conflicts:

	neutron/tests/unit/bigswitch/test_servermanager.py

Closes-Bug: #1348766
Change-Id: I512d01f9bb91b208dd58883d2464951ecc6748e1
(cherry picked from commit ec65fe5e2fb82850905c87587ebec2436227ceeb)
Merge branch 'stable/icehouse' of github.com:bigswitch/neutron into stable/icehouse