Allow skipping/postponing the dn search

by refactoring the _LDAPQuery class:

  - slightly refactor _LDAPQuery.execute by splitting it into
    a new method _LDAPQuery.execute_cache, which does the real work
    of searching and caching, and a replacement _LDAPQuery.execute
    which will skip the call to execute_cache when filter_tmpl
    is empty.

  - add a search_after_bind parameter which is honored by
    Connector.authenticate()
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.