Description of problem:
e2e-gcp job fails due to invalid resource.sizeGb 
For example: https://prow.svc.ci.openshift.org/view/gcs/origin-ci-test/pr-logs/pull/24382/pull-ci-openshift-origin-release-3.11-e2e-gcp/927

Version-Release number of selected component (if applicable):
origin-release-3.11

How reproducible:
Every time seemingly (https://prow.svc.ci.openshift.org/job-history/origin-ci-test/pr-logs/directory/pull-ci-openshift-origin-release-3.11-e2e-gcp)


Steps to Reproduce:
1. Wait for pull-ci-openshift-origin-release-3.11-e2e-gcp
2. ??
3. Error

Actual results:
template pod "e2e-gcp" failed: the pod ci-op-ycip5r0h/e2e-gcp failed after 3h3m15s (failed containers: setup): ContainerFailed one or more containers exited

Container setup exited with code 2, reason Error
---
---------------------- 0.03s
/usr/share/ansible/openshift-ansible/playbooks/gcp/openshift-cluster/build_image.yml:27 


Failure summary:


  1. Hosts:    localhost
     Play:     Launch image build instance
     Task:     Create the image instance disk
     Message:  Unexpected response: ({u'domain': u'global', u'message': u"Invalid value for field 'resource.sizeGb': '10'. Requested disk size cannot be smaller than the image size (20 GB)", u'reason': u'invalid'}). Detail: Traceback (most recent call last):
                 File "/tmp/ansible_gce_pd_payload_kxnqjw/__main__.py", line 237, in main
                   snapshot=lc_snapshot, ex_disk_type=disk_type)
                 File "/usr/lib/python2.7/site-packages/libcloud/compute/drivers/gce.py", line 5063, in create_volume
                   data=volume_data, params=params)
                 File "/usr/lib/python2.7/site-packages/libcloud/common/base.py", line 772, in async_request
                   response = request(**kwargs)
                 File "/usr/lib/python2.7/site-packages/libcloud/compute/drivers/gce.py", line 121, in request
                   response = super(GCEConnection, self).request(*args, **kwargs)
                 File "/usr/lib/python2.7/site-packages/libcloud/common/google.py", line 808, in request
                   *args, **kwargs)
                 File "/usr/lib/python2.7/site-packages/libcloud/common/base.py", line 637, in request
                   response = responseCls(**kwargs)
                 File "/usr/lib/python2.7/site-packages/libcloud/common/base.py", line 159, in __init__
                   self.object = self.parse_body()
                 File "/usr/lib/python2.7/site-packages/libcloud/common/google.py", line 288, in parse_body
                   raise InvalidRequestError(message, self.status, code)
               InvalidRequestError: {u'domain': u'global', u'message': u"Invalid value for field 'resource.sizeGb': '10'. Requested disk size cannot be smaller than the image size (20 GB)", u'reason': u'invalid'}


Expected results:
Expected successful "Launch image build instance" step