Fixed length calls (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.

(cherry picked from commit 7c2fede9fc355db2b108e2fc720218e5f2c597c3)
Merge pull request #119 from tboerger/feature/roxy-fixed-length

Fixed length calls (bnc#879657)