devstack to `git pull sushy-tools` if required
devstack to `git pull sushy-tools` if required

This change to devstack plugin code checks if `sushy-tools`
has been requested to be pulled from git rather than pypi.
This is a prerequisite for `sushy-tools` CI job to test the
Redfish emulator against Ironic conductor.

Change-Id: I548f5a19a266436edbb874ea14af67db711240be

openstack/ironic,openstack/ironic