ZFSSA iSCSI driver should support extra specs

Support for extra specs at volume creation time,
would allow more flexibility to create custom volumes.
The following scoped keys are supported:
-zfssa:volblocksize
-zfssa:sparse
-zfssa:compression
-zfssa:logbias

Closes-Bug: #1379403
DocImpact

Change-Id: I8768d07f19147dc2544bebc0dbf3392313838e8f
Merge "ZFSSA iSCSI driver should support extra specs"