Allow admin to delete default security groups

Previously there was no way to delete a default security groups which
isn't ideal if you want to clean up after deleting a tenant. This patch
allows default security groups to be deleted by the admin.

Fixes bug 1175393

Change-Id: I2214c7dabf0f2ec960ce10ebbbcdc513bc73664c
Merge "Allow admin to delete default security groups" into stable/grizzly