Mjulian/samba4 dovecot aliaslogin

Merge with caution!

This branch has been created from this pull requested branch => https://github.com/Zentyal/zentyal/pull/1313

Once the mail schema has been added to Samba4 LDAP, with this branch, we would be able to make available for users a mail alias IMAP/POP login against Dovecot (this should be a new feature as now, we handle both alias and mail in the same LDAP object).

We are using the extended schema attribute "otherMailbox" to login, and later on, retrieve the correct attributes using  its unique "mail" attribute to query again the LDAP.