Adds handling of running ironic API under Apache as WSGI app to
devstack plugin.
New variable IRONIC_USE_MOD_WSGI (False by default) is added.
Another new variable IRONIC_WSGI_DIR (default is distro specific) is
also added, which specifies location for WSGI scripts.
Change-Id: I9c5ad56e1acd292ff0f9cc9b460125fc420abda5
Closes-Bug: #1513005