Fixed length calla (bnc#879657)

It is possible that crowbar bails out while apply a pacemaker proposal
with an exception that length is not a method of NilClass. With this fix
it should be fine now.

https://bugzilla.novell.com/show_bug.cgi?id=879657

Backport of https://github.com/crowbar/barclamp-pacemaker/pull/118
(cherry picked from commit 7c2fede9fc355db2b108e2fc720218e5f2c597c3)