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