Remove snet hosts file hack

A previous commit overlooked the fact that glance exposes
swift_store_endpoint_type, which allows us to use specify publicURL,
internalURL, etc.  This change leverages that configuration option
which makes this change more generic so it can be used with other
non-RS swift installations.
Add comment to user_variables.yml file

This small change adds a comment to user_variables.yml stating that the
default value for glance_swift_store_endpoint_type will cause glance to
communicate with swift over ServiceNet and to use publicURL to switch
to communicating over the public network.
Merge pull request #48 from mattt416/snet_update

Remove snet hosts file hack