Add Kerberos Negotiate auth plugin

This plugin depends on the proper configuration of mod_auth_kerb

The mod_auth_kerb plugin should be configured with a <Location> directive
like the folowing:

<Location /idp/login/krb/negotiate>
  AuthType Kerberos
  AuthName "Kerberos Login"
  KrbMethodNegotiate on
  KrbMethodK5Passwd off
  KrbServiceName HTTP
  KrbAuthRealms $REALM_NAME
  Krb5KeyTab $KEYTAB_NAME
  KrbSaveCredentials off
  KrbConstrainedDelegation off
  Require valid-user

  ErrorDocument 401 /idp/login/krb/unauthorized
</Location>

Signed-off-by: Simo Sorce <simo@redhat.com>