Description of problem:
After registering a system and subscribing to some subscriptions, headpin system subscriptions command couldn't display entitlement certificate serial numbers.

Version-Release number of selected component (if applicable):
katello-headpin-all-0.1.122-2.el6.noarch
katello-headpin-0.1.122-2.el6.noarch
katello-cli-headpin-0.1.13-2.el6.noarch
katello-cli-common-0.1.32-2.el6.noarch

How reproducible:
always

Steps to Reproduce:
1.Register a system to SAM.
# headpin -u admin -p admin system register --name=testname --org=org1 --environment=env1
Successfully registered system [ testname ]

2.Consume a subscription.
# headpin -u admin -p admin system subscribe --org=org1 --name=testname --pool=8ac2851934b491950134b49feb28003b
Successfully subscribed System [ testname ]

3.List certificate serial numbers.
# headpin -u admin -p admin system subscriptions --org=org1 --name=sam-120106095640.redhat.com --serials

Actual results:
Serial parameter cannot be used with available

Expected results:
Entitlement certificate serial numbers should display successfully.