BSN: Don't update hash value without hash header

Don't update the hash value in the database if the
response from the backend does not have a hash header.

Closes-Bug: #1348766
Change-Id: I512d01f9bb91b208dd58883d2464951ecc6748e1
Merge branch 'stable/icehouse' of github.com:bigswitch/neutron into stable/icehouse