Partial revert of 257b10d37405e28f3bf292e1041bf2c3010c02c6

* .../jenkins_job_builder/config/swift-functional.yaml: Revert change of
devstack-gate hook use in the swift functional test. The functional test
must run after devstack runs so they should use the post_test_hook and
not the gate_hook.

Change-Id: Ia980be99f98dcd4020de83080f960b8f43ed6447