Handle maxclonepervolume/node limits in SF driver.

The SolidFire cluster has a max simultaneous clones per volume and node
limt that we weren't accounting for.  Add some logic to detect this situation
and loop until enough simultaneous clones are completed to allow processing
of the next one in line.

Fixes bug: 1129810

Change-Id: Id63c1b142a81df5d9d2c4e39fb821b03ebac63a2
Merge "Handle maxclonepervolume/node limits in SF driver."