man: ifcfg-tunnel updates

Document support in wicked for sit, ipip and gre tunnels.
samples: xml config nodes for sit ipip and gre tunnels
Merge pull request #384 from kmroz/tunnel-doc-update

man: ifcfg-tunnel updates
Merge pull request #385 from kmroz/tunnel-samples

samples: xml config nodes for sit ipip and gre tunnels