Added unit test cases for _is_share_eligible in NFS driver.
Added unit test cases for _is_share_eligible in NFS driver.

The generic NFS driver (under cinder/volume/drivers/nfs.py) has
a method called _is_share_eligible, which checks if a given NFS
share meet user specified conditions, such as:
  1. The ratio of actual space (used_space / total_space) is
     less than 'nfs_used_ratio'.
  2. There is enough space is available for the new volume.
This patch adds the corresponding unit test case for it.

Change-Id: I4489cbbca0a1a75a1144a87628843baf0a92f8a8
Closes-Bug: #1298414

eharney/cinder,cloudbase/cinder,Nexenta/cinder,tlakshman26/cinder-https-changes,manojhirway/ExistingImagesOnNFS,CloudServer/cinder,j-griffith/cinder,scality/cinder,tlakshman26/cinder-https-changes,petrutlucian94/cinder,saeki-masaki/cinder,Thingee/cinder,alex8866/cinder,blueboxgroup/cinder,ge0rgi/cinder,scottdangelo/RemoveVolumeMangerLocks,leilihh/cinder,bswartz/cinder,mahak/cinder,hguemar/cinder,Datera/cinder,Nexenta/cinder,scality/cinder,Hybrid-Cloud/cinder,dims/cinder,j-griffith/cinder,takeshineshiro/cinder,manojhirway/ExistingImagesOnNFS,julianwang/cinder,potsmaster/cinder,blueboxgroup/cinder,Thingee/cinder,mahak/cinder,nexusriot/cinder,scottdangelo/RemoveVolumeMangerLocks,winndows/cinder,Accelerite/cinder,julianwang/cinder,openstack/cinder,abusse/cinder,Hybrid-Cloud/cinder,apporc/cinder,apporc/cinder,hguemar/cinder,openstack/cinder,Datera/cinder,abusse/cinder,sasukeh/cinder,nikesh-mahalka/cinder,JioCloud/cinder,Paul-Ezell/cinder-1,dims/cinder,phenoxim/cinder,NetApp/cinder,Thingee/cinder,github-borat/cinder,takeshineshiro/cinder,leilihh/cinder,tlakshman26/cinder-bug-fix-volume-conversion-full,winndows/cinder,potsmaster/cinder,rakeshmi/cinder,sasukeh/cinder,bswartz/cinder,NetApp/cinder,rakeshmi/cinder,eharney/cinder,Accelerite/cinder,alex8866/cinder,duhzecca/cinder,redhat-openstack/cinder,tlakshman26/cinder-bug-fix-volume-conversion-full,phenoxim/cinder,CloudServer/cinder,Paul-Ezell/cinder-1,nexusriot/cinder,tlakshman26/cinder-new-branch,Akrog/cinder,tlakshman26/cinder-new-branch,redhat-openstack/cinder,Akrog/cinder,duhzecca/cinder,cloudbase/cinder,nikesh-mahalka/cinder,petrutlucian94/cinder,saeki-masaki/cinder,github-borat/cinder,JioCloud/cinder