Redefine instance.security_groups as list

This patch is the 2nd time I am patching Horizon to fix a Django
1.7 patch: the rework of the tests seems to have dropped the
fix, so I have to do the work again. Reviewer: please make sure
this doesn't happen again, and that we don't go back to .first().
Please see: https://review.openstack.org/111934/

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: I92ae19e937e0e3a482fa424585b926e853c4ce8b
Closes-Bug: #1374919
Merge "Redefine instance.security_groups as list"