Add oneshot? method to service objects

This allows barclamp to specify that there should not be a second
chef-client run in case of failures. This is needed for one-shot
barclamps, where a second chef-client run won't do anything anymore and
therefore will always result with a success.

See discussion at
http://lists.us.dell.com/pipermail/crowbar/2013-June/003302.html
Merge pull request #571 from vuntz/oneshot-chef-run

Add oneshot? method to service objects