Allow ceilometer-api to run under mod_wsgi

If CEILOMETER_USE_MOD_WSGI is True then the API app will
run under mod wsgi. The default is false (for now).

The changes are modeled on keystone's use of apache.

Note that these changes are dependent on
https://review.openstack.org/#/c/121823/ in ceilometer.

Using mod_wsgi allows the ceilometer api to handle "concurrent"
requests. This is extremely useful when trying to benchmark
various aspects of the service.

Change-Id: I4c220c3b52804cd8d9123b47780a98e0346ca81e
Merge "Allow ceilometer-api to run under mod_wsgi"