Add support for configurable pre and post hooks.

Adds 3 hooks to the devstack-vm-gate-wrap.sh script so that we can
make use of this script for non-devstack type gating tests.

The new hooks are:
 -pre_test_hook (optional, only executed if defined)
 -gate_hook (runs the devstack-vm-gate.sh as a default)
 -post_test_hook (optional, only executed if defined)

Change-Id: I9d2435780a23a04a8b34b2d885125132a7e4f16a