Fix error raising in security groups method

Fix error raising in security groups method

In case there were security groups not belonging to tenant on port
_get_security_groups_on_port would try to raise exception but fail
trying to index set.

This patch simply joins the whole set as a string and inserts it
into the standard SecurityGroupNotFound exception.

No new exception types, no string freeze violations.

Co-Author: watanabe.isao <zou.yun@jp.fujitsu.com>
Co-Author: Jacek Swiderski <jacek.swiderski@codilime.com>

Change-Id: I039ea57269dc53ced8dece0985f33ce9ae7eab17
Closes-Bug: #1373816