Make LoadBalancer nested stack template configurable

Reviving work originally posted by Angus Salkeld under
https://review.openstack.org/#/c/21405/

This is to enable the various distros to easily override the
hard-coded default nested template (which currently only supports
Fedora 17) with a template approporate to their distro, and
enables deployers to specify their own template (although the
latter use-case will probably be catered for via a provider
template implementation long-term).

Conflicts:
	heat/engine/resources/loadbalancer.py

Partial-Bug: #1234375
Co-Authored-By: Angus Salkeld <angus@salkeld.id.au>
(cherry picked from commit 746a0ed4ad2273d0e67b896efbb6d7a69f9f4c77)
Change-Id: I0a4eccb23b2eebbf31b7aabf0fe85ab31f0ca971