Allow value "plain" for $ldap_method in init.pp

The regexp validation of $ldap_method only allowed the values ssl and tls. This way, a plain authentication against a LDAP server was impossible without a manual change of the gitlab.yml file.
Merge pull request #100 from svengerlach/feature/ldap-method-allow-plain

Allow value "plain" for $ldap_method in init.pp