Refactored kernelCmdLinePackages to rely on .proc.cmdline agent (bnc#889365)

A new SCR instance is opened but it's not closed, this looks like a memory leak (see https://github.com/yast/yast-yast2/pull/274/files#diff-03c6171b2e9fe36e9b596efc92e8059bR15 how to close it).