Enable LDAP authentication from Puppet manifest

Until now, one could provide a different passdb_backend that tdbsam,
but the specific fields for the ldapsam backend were not present.
This PR allows one to configure the ldapsam from the puppet manifest.
Enable a check before resetting the secrets.tdb ldap bind dn password

A check is now applied before running the smbpasswd -w to verify that
we are not resetting the password to it's same value, avoiding useless
reboot of the service. The tdb-tools package needed to be installed so
it could be done
Merge pull request #1 from Spredzy/ldap_backend

Enable LDAP authentication from Puppet manifest