Fix NetApp clone from glance failure

Fix NetApp clone from glance failure

When glance and cinder are backed by the same NetApp DOT flexvol
container, exposed as an NFS share, a direct clone is attempted
of the backing files rather than a copy.  With the advent of
kilo 2015.1.1, this clone operation fails with the message:
"Image cloning unsuccessful ... need more than 2 values to unpack."

The issue here is that the clone path code is calling the parent
nfs class _is_share_eligible() method to see if there is enough
space on the destination flexvol and this method calls _get_capacity_info()
expecting a three-tuple to be returned.  However, commit
e52f304313efc695f7dd89c222041bffd53c131a deliberately changed the return
values of _get_capacity_info() within the NetApp drivers to return
a 2-tuple, based on API calls to obtain capacity information from
the NetApp storage arrays. That commit missed that this method
would also be invoked by the parent nfs class _is_share_eligible()
method, which expects a different signature.

The fix in this commit is to replace the call to the parent
_is_share_eligible() method with a more appropriate NetApp class
specific _share_has_space_for_clone() method.  This new method
uses the NetApp version of _get_capacity_info to get space information
from the arrays via NetApp apis and expects a 2-tuple to be returned
from this method.

Change-Id: Ib0c69e5ea7b32d17930fe0bdcdb9357fd4e4a244
Closes-bug: 1490845