Use :install and not :upgrade for installing packages

As agreed during a community call, we want to use :install to allow
upgrades being controlled instead of automatically installed.
Avoid unneeded platform checks inside SUSE-specific paths
Only define nova-novncproxy service once

We were defining this in the different code paths, depending on the
platform.
Merge pull request #162 from vuntz/use-install-action

https://github.com/crowbar/barclamp-nova/pull/162

Merged by devtool for cloudsleuth