add some negative tests for security group:

- test_create_default_security_group
- test_create_sg_rules_with_non_existent_sg

Change-Id: Ica468bc0c41845050505a636a2add47b586fcac3
Fix a minor error in method _get_alternative_flavor

Current ServerDiskConfigTestJSON._get_alternative_flavor
defined in file test_disk_config.py cannot get the correct
alternative_flavor. The condition used to decide whether
the flavor_alt should be used can never return true because
a integer never equals to a string.

Change-Id: Ib4f06d0a93580d84fcea0f475695f2c9fd34856b
Merge "add some negative tests for security group:"
Merge "Fix a minor error in method _get_alternative_flavor"