This side-effect was removed in "Pass connection helper as a constructor
parameter, not as class property." commit on slapos.core repository
(hash = c2d5a1eb5d256ef580b1afd34633413adee23e8c).

Closes the gap between command-line arguments and configuration file.
Also, make key_file and cert_file optional, as they are only needed by
slap when master_url uses the "https" scheme.

We maintain time stamps locally.

0.23 was released instead of 0.22.

argparse is available from python2.7, but a lot of python2.7 compilations does
not provide it.

As users expect package to work by default, in case if python used to install
package does not contain argparse, add it to install_requires without requiring
any intervention, because default invocation shall work.

It will allow to minimize common dependency and require ZODB3 module.