Cleanup class resources for large-ops scenario

Large Ops scenario is design to test the bulk creation of VMs while multiple
VMs already exist in tenant.
Due to some refactoring in Tempest scenario, the VMs are now deleted at the end
of the individual test cases.

Removes usage of addCleanup() from large-ops scenario.

Adds Class level method - addCleanupClass to cleanup resources at the end of
the test class.

Closes-Bug: #1395759
Change-Id: I612fcfab83a2046b8e597e009e385d7918f943b9
Merge "Cleanup class resources for large-ops scenario"