Add SITE_PACKAGES variable to horizon extend_start
Add SITE_PACKAGES variable to horizon extend_start

This change adds a missing export. Without this change the horizon
dashboards are all broken becuase the functions are able to
correctly copy the dashboards plugins to the appropriate locations.

Signed-off-by: Kevin Carter <1d9a750a4fc7026fb02029431dc8a87d24487135@redhat.com>
Change-Id: I5fa3bb41403620e8bd9521e3c0e61570e202148f

openstack/tripleo-common,openstack/tripleo-common