Fix documentation for OPENSTACK_ENDPOINT_TYPE

The default endpoint type is "publicURL", despite
what the official Horizon documentation says.
Add attribute for OPENSTACK_ENDPOINT_TYPE

This setting controls which endpoints Horizon will use from
the Keystone service catalog. The default attribute will be
set to `internalURL`, which should probably only be changed
if Horizon is running external to the OpenStack environment.
Merge pull request #107 from DavidWittman/grizzly

Add support for setting OPENSTACK_ENDPOINT_TYPE