Fix user creation from incomplete LDAP data

In commit 23aa3e4 only the creation of users with missing passwords has been fixed.
Unfortunately there are cases in which users from LDAP are also missing some other
attributes like the full name/display name or an email address.

Since most of these attributes are optional, users will now have an email address
constructed from the user name and `localhost` if the email address is missing.
The full name attribute has been made optional.

Refs #661, refs Graylog2/graylog2-web-interface#905