Add benchmark tests for glance

The patch adds the following benchmark tests for glance:
* Add and delete image
* Boot the several instances from added image

Also some changes are added related with using
fake Glance client the for following unit tests:
* test_generic_cleanup in tests/benchmark/test_runner.py
* test_snapshot_server in tests/benchmark/scenarios/nova/test_servers.py

Implements: blueprint benchmark-scenarios
Change-Id: I31471a91455bd20eea000a66c59320178455cb50
Don't require glance to use OpenStack server provider

Currently the openstack server provider fails if glance is not present
in the service catalog, even if the user only wishes to use pre-existing
images.

This is a problem when using certain *cough* cloud providers that don't
expose glance.

Change-Id: Ib91be7e9396190045d82c3f0a059c92a73374499
Closes-Bug: #1277150
Don't initalise deployment engine when starting a task

In some cases initialsing the deployment engine can lead to
authentication against external services - eg openstack server provider.
This is unnecessary when the deployment is already complete.

Change-Id: Ibe06a6acf8396023059292a9e5804616ed28e766
Closes-Bug: #1277342