Infinidat: Get rid of pylint errors due to private lib

Infinidat has a requirement for a vendor lib on the host
which appears to include a package called 'capacity'. Its
use and properties appear to be the same as the units
package.

When loaded and this package is not available it is set to
None, such as when running tests on generic nodes. This
results in pylint errors:

"Module 'capacity' has no 'GiB' member"

On failure to load, this aliases the units module as
capacity to prevent these errors.

Change-Id: Ia522b1c6d866acaa39439ca86c4a7bbfe4e0f70a

cinder/volume/drivers/infinidat.py

@@ -48,7 +48,7 @@ try:
     from infi.dtypes import wwn
     import infinisdk
 except ImportError:
-    capacity = None
+    from oslo_utils import units as capacity
     infinisdk = None
     iqn = None
     wwn = None