Set invalid (but non-empty) password for accounts synced from LDAP

Accounts which have been synced from an LDAP directory had an password by default.
Unfortunately our validation didn't take this into account and the automatic creation
of the user account in Graylog2 subsequently failed.

User accounts synced from an LDAP directory will now get an invalid but non-empty
password. This prevents the failing of our validation but still prevents users from
logging into Graylog2 when the LDAP server isn't available.

Fixes #661