Update EMC SMI-S iSCSI Driver

Update EMC SMI-S iSCSI Driver

Refactor EMC SMI-S based iSCSI driver.  There are lots of common code
that can be shared between iSCSI and FC drivers using SMI-S.  Break
the original emc.py into two files: emc_smis_common.py and
emc_smis_iscsi.py.  Also added support for get_volume_stats() and
create_cloned_volume().  get_volume_stats() will be revisited after
winston-d's changes.  copy volume<->image will be revisited after
avishay-il's changes.

Change-Id: I90dc3599ec2fd5bb86a3e988111a8aee5de7efea