Support SSH public keys stored in LDAP and other directories

At the moment, Stash only supports SSH keys that the user has added directly to their account on Stash.

In an enterprise environment, however, it is possible for LDAP to store public keys centrally, thus avoiding duplication of effort on the user's part.

It should be fairly straightforward for Stash to pull those keys out of LDAP when a sync operation is performed. Furthermore, if/when Crowd is extended to support more attributes from LDAP, including SSH keys, Stash would then already have the logic to sync keys from Crowd as well.