admin enforcing an empty authorized_keys

Previously we were only processing the ssh_key field if it was
a populated array.  Otto had a use case where a user wanted
their key revoked for a period of time due to a security concern
while keeping their account.  These changes basically mean that
we enforce an empty authorized_keys file if the ssh_key array
is empty.

Change-Id: I699e63cba1a399a33add185847e36e4b68ba8a5f
Merge "admin enforcing an empty authorized_keys" into production