added ldap_filter to ldap auth sources to filter login lookups more specific using RFC 2254 filters

added LDAP filter syntax validation

added ldap_filter tests

small cleanup: now using blank? to check ldap_filter content, validate function now always returns boolean

removed redunant returns, optimized validation use

fixed typo, fixed string in template
Merge branch '863-ldap-filter' of https://github.com/endyman/foreman into 863-ldap-filter