Add unit tests for vm benchmark scenarios

Added following tests

test__boot_server_with_sec_group
test__boot_server_with_sec_group
test__boot_server_with_similar_sec_group

Created method to remove repeated
assert_called_once_with calls

Change-Id: I65b2e398826a5883d033eb8b5214dd41ceea3063
Implements: blueprint improve-unit-test-coverage-rally
Merge "Add unit tests for vm benchmark scenarios"