large ops test should be derived from OfficialClientTest

Scenario test_large_ops runs on gate-tempest-dsvm-large-ops
& gate-tempest-dsvm-neutron-large-ops jobs.

This test case boot VM with security grp by requesting the same from
Nova. Nova can behave as proxy in case neutron is enabled.

This is needed to migrate this test from official
client to tempest client as _create_security_group() fucntion
which request nova to create sec grp can not be used via
NeutronScenarioTest.

Partially-implements: bp:tempest-client-scenarios

Change-Id: I7b69811b49caf3aec5983e27862ce955aadf958a
Merge "large ops test should be derived from OfficialClientTest"