Rename "Interface" to "Availability".

Interface is what the identity v3 docs call it, but that collides with an
important concept from Go-the-language. Renaming it to "Availability" because
that's more descriptive.
Merge pull request #196 from smashwilson/rename-interface

Rename "Interface" to "Availability".
add ApplyDefaults method for EndpointOpts
fix import path