Client: add --only-important command line option

Add the new --only-important (-i) command line option. With this option
bcfg2 will only configure the important entries and exits clean
afterwards.
Client: support dryrun mode and only-important

Add support for displaying the skipped entries in dryrun mode when using
the --only-important command line flag.
Merge pull request #185 from AlexanderS/only-important

Add only important flag