Remove credential validation from creation

FileDepotNfs.create!(:uri => "nfs://test") fails because of missing credentials
https://bugzilla.redhat.com/show_bug.cgi?id=965889
Add #verify_credentials methods to appropriate FileDepot subclasses

https://bugzilla.redhat.com/show_bug.cgi?id=965889
Change FileDepot ::SUPPORTED_DEPOTS to #supported_depots

Build a hash of subclass => display_name for all descendants
Update drop-down to to get the correct uri_prefix based on the model

https://bugzilla.redhat.com/show_bug.cgi?id=965889
Add support_case and depot_name to the edit screen where appropriate

https://bugzilla.redhat.com/show_bug.cgi?id=965889
Add reverse relationship for MiqServer#log_file_depot

https://bugzilla.redhat.com/show_bug.cgi?id=965889
Create log_file_depot using relationships

https://bugzilla.redhat.com/show_bug.cgi?id=965889
Simplify all the things

https://bugzilla.redhat.com/show_bug.cgi?id=965889