Update 3PAR driver session management

The 3PAR REST API server has a limited number of active sessions.
This change to the driver removes the ability of keeping the
REST API session alive for the life of the driver.  Now on every
driver entry point, which is synchronized, we login to the 3PAR
and when the work is complete, we log out of the 3PAR.  This has
the side affect of more overhead to the 3PAR but helps eliminate
failed commands due to maximum sessions reached to the 3PAR.

This patch also changes the locking to be the same lock for all
driver entry points and adds locking around the volume stats.
This patch also migrates all of the client code access to the common
class, which simplifies the drivers.

Change-Id: Ibcec4cf1781262521ccbdf99c4ba4167634a09c4
HDS Cinder Driver. Rev #1

blueprint hds-hus-iscsi-cinder-driver
This is the first rev of Hitachi Data Systems Cinder iSCSI driver.
This driver works with HUS (df850) array.
This driver contains all the base-line features specified for Havana release.
Amended into this submission are changes from code-reviews.

Docimpact: Bug #1180648

Change-Id: Ia27d076443b10da2c653456f9292dd192362b853
Merge "HDS Cinder Driver. Rev #1"
Merge "Update 3PAR driver session management"