Redefine instance.security_groups as list

The two tests modified here were incorrectly defining
instance.security_groups as a single value instead of a list.

Apparently Django 1.7 is no longer happy trying to iterate something that
is not an iterable.

The other test_instance_details_*() were already doing the correct
thing so just copy over the logic.

Change-Id: I3ea2525927f9adbc1f000a03b5aedfbcb2595c02
Merge "Redefine instance.security_groups as list"