Issue#110 : specify endpoint interface configuration

add endpoint type configuration:
if specified, services use the given url : adminURL, internalURL or publicURL
if not specified, default is publicURL
I think it would be best to put the default value in the config itself, like https://github.com/mael-morel/vagrant-openstack-provider/blob/issue%23110/source/lib/vagrant-openstack-provider/config.rb#L254.

This way, if at some point, another method need to access this option, it will not need to copy the logic in this line