FR: Handle multiple key files

[jhmartin]

It would be useful for key maintenance to be able to place one key per file in the S3 directory, rather than uploading a single combined authorized key file.

The key manager would then search the defined location for say all files matching a pattern (say, *.pub) and merge them together before emitting them to SSHD. In this manner adding a key is as simple as copying myusername.pub into the relevant S3 directories, rather than modifying a monolithic file.