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
Remove not used attribute _schema_file

This was actually a remaining of [1].

[1]: https://review.openstack.org/#/c/115918

Change-Id: Idcf589fa9d89c2969914ef0c8aee27a22325470e
Adapt negative testing documentation

 - Removes the json file documentation
 - Removes XML part

Change-Id: I6a7fdcb12c84189f9adde6f83fee8b303c28a4ee
Merge "Remove not used attribute _schema_file"
Merge "Adapt negative testing documentation"
Merge "Cleanup class resources for large-ops scenario"