Allow skipping/postponing the dn search

  - slightly refactor _LDAPQuery.execute taking the real search
    into execute_cache, which will not be called if filter_tmpl
    is empty

  - add the search_after_bind parameter to force a base search
    after binding
Use string.Template instead of % formatting
Allow declarative configuration of the module

via named entries in pastedeploy .ini file
Update documentation

use string.Template style replacement markers in existing examples
and add declarative configuration description and example.