Make sure we have tripleo-heat-templates at tuskar startup

If we don't then tuskar startup cannot proceed, with an appropriate
error message. A conf var is added: CONF.tht_local_dir that points
to the local path in which the templates exist. A relevant note
was added to INSTALL.rst

Implements: blueprint retrieve-ooo-templates
Change-Id: I7cb1151f35aef00f416a7d06be6229553fbe9d22