Michal Vasko

Fixes of `systemd` lens as described in the commit messages, tests included.

Simply port #1337 over.

It is possible for server to modify its supported schemas ([YANG 1.1 RFC](https://tools.ietf.org/html/rfc7950#section-5.6.4)) and the client may also want to support the new context. *libnetconf2* does not allow this currently....