Made DNS configuration more robust, and support systemd-networkd

[asheplyakov]

In addition to supporting systemd-resolved this PR makes DNS configuration more fool proof, i.e. DNS related tasks now

• Skip unnecessary network reconfiguration on clients
• Remove resolv.conf files from /etc/net/ifaces/IFACE directories for real
• Perform basic validation of the /etc/resolv.conf file (check that the master DC is listed)
• Don't fail if openresolv and/or etcnet packages are not installed
• Don't reconfigure eth0 interface if it is not managed by etcnet