NetApp E-Series: Fix instance live-migration with attached volumes

NetApp E-Series: Fix instance live-migration with attached volumes

Currently, live migrations of instances with attached volumes that live
on a NetApp E-Series backend will fail and break connectivity to the
guest. This patch adds the 'netapp_enable_multiattach' configuration
option that enables multiattach operations with the E-Series driver.
It defaults to allowing these operations but needs to be configurable
since allowing for multiple attachments imposes a limit of 256 volumes
on the backend due to how multiple attachments must be managed by
E-Series.

Multiattach operations are enabled by mapping volumes to an E-Series
host group on the backend called 'cinder-host-group'. Host groups can
only have 256 mappings at a time and so we must limit the number of
volumes in order to guarantee any volume created could then be attached.

Closes-bug: 1433825
Change-Id: Icb73f5b65b12decbd7b7f21c2fcec6fe700eaf59