get_instance_security_groups() fails if no name on security group

The following patch fixes a bug in get_instance_security_groups() where
it tries to access security_groups['id'] but security_groups is a string.
This patch also removes some stubs that were not needed in a few unit test.

Fixes bug 1182738

Change-Id: Ia64db2e40d4a0e163344b4234112dbde71b1e132
Merge "get_instance_security_groups() fails if no name on security group"