${GUAC_USERNAME} name become domainname\username multiple LDAPS

As per documentation for example we are configuring guacamole to use multiple LDAP's  in `ldap-servers.yaml` like below, with `match-usernames` option

- hostname: dc1.example.net
  user-base-dn: ou=Users,dc=example,dc=net
  username-attribute: sAMAccountName
  search-bind-dn: cn=Guacamole,ou=Service Users,dc=example,dc=net
  search-bind-password: SomePassword!
  match-usernames: COMPANYA\\(.*)
- hostname: dc2.example.net
  user-base-dn: ou=Users,dc=example,dc=net
  username-attribute: sAMAccountName
  search-bind-dn: cn=Guacamole,ou=Service Users,dc=example,dc=net
  search-bind-password: SomePassword! 
  match-usernames: COMPANYB\\(.*)

In this case, to login i have to use `domain\username`. so after successful login ${GUAC_USERNAME} =>  `domain/username`.

Where as for single LDAP configuration in `guacamole.properties` ${GUAC_USERNAME} => `username`.

This is causing us to use ${GUAC_USERNAME}  as username in RDP session connection.

How about doing, when user log-in into Guacamole extract domain name and username from DOMAIN\username, load into new variables

1. 1. Put username into  ${GUAC_USERNAME_ID}
   2. Domain name into ${GUAC_USERNAME_DOMAIN}

so we can use different variables when multiple LDAP's enabled.

I can able to fetch username using configuration in guacamole.properties `ldap-user-attributes: sAMAccountName` into ${LDAP_SAMACCOUNTNAME} 

So proposing just extract domain name into ${LDAP_DOMAIN_NAME} is enough. I have raised pull request for this https://github.com/apache/guacamole-client/pull/931