Ldap email

Added a config option in gitlab.yml.example to set a default email domain in the event that LDAP does not provide email addresses. Currently using this model at Rutgers University. If the "email_domain" option is set and the user does not have an email associated with them already in LDAP, an email address is formulated using the formula "_ldap_nickname@_config_email_domain", which is reflected in auth.rb.  This option can be disabled by setting it to null.