Add support for setting OpenStack endpoint type

Alternate service endpoints can be used by setting
`knife[:openstack_endpoint_type]` or passing
`--openstack-endpoint-type` on the command-line.

Valid options are "publicURL" (default), "internalURL", or "adminURL"
Add rspec test
Merge pull request #87 from opscode/adamed/knife-395

KNIFE-395: Add support for setting service endpoint type