[predicates] fix predicates that erroneously evaluate the entity passed as a parameter as a boolean, instead of testing its existence in the keyword arguments -using the kwargs.get('entity') idiom instead of 'entity' in kwargs or similar- ; closes #2424288
[ldap feed] fix error since with read security activated, password value is not selecteable (closes #2406597). Also add a note in the book about packages required to connect to an ldap server