Update Apache + mod_wsgi deployment mechanisms

Added a global toggle for enabling HTTPD + mod_wsgi for services
that default deploy to running under Apache. When the variable
``ENABLE_HTTPD_MOD_WSGI_SERVICES`` is set to ``True`` any service
that recommends deploying under HTTPD + mod_wsgi, will be run
under Apache.

If ``ENABLE_HTTPD_MOD_WSGI_SERVICES`` is set to ``False`` the
any service that is defaulted to running under HTTPD + mod_wsgi
will (if capable) be run in an alternate deployment strategy (e.g.
eventlet).

Updated Swift and Keystone to have individual toggles for deploying
under HTTPD + mod_wsgi. This is done to allow for gate to run on
the services under mod_wsgi where appropriate. Toggles are
``KEYSTONE_USE_MOD_WSGI`` and ``SWIFT_USE_MOD_WSGI`` and are both
defaulted to "False" (do not deploy under HTTPD + mod_wsgi).

Change-Id: Id3b121b8f1cde369d184b586e0d875bdbda34813
Merge "Update Apache + mod_wsgi deployment mechanisms"