Added proper support for RHEL8 with NetworkManager (#306)

* Better RHEL 8/CentOS 8 NetworkManager support

* rule-<device> is not parsed by nm-settings-ifcfg-rh plugin
  instead it is read directly from the ifcfg-<device> file
* "nmcli networking off && nmcli networking on" isn't the best idea
  as it completly cuts off the network without enabling it again
  when running with a puppet master from the network because it isn't
  called in one command.
* use "nmcli reapply <device>" to configure individual interface

* Added new tests

* to thest rules for rhel8
* don't set manage_all_nic bevahiour in params.pp

* fixed all broken tests introduced in the last two commits

* migrated all RHEL tests to use concat instead of file
* improved some resource behaviour (check if not defined)

* added more tests for RHEL8

* now a test checks if the networkmanager is running
* versioncmp for rhel >= 8 is now used

* enhanced rhel8 rule test

* fixed puppet-lint error (two soft tabs not used)

* updated readme and metadata

* prevent duplicate declarations in defined resource

* reverted change to concat

* now resource "interface" has additional parameter
  to set ip rules for NetworkManager

* README information about RHEL + NetworkManager iprules