Refactor PureISCSIDriver into base and iSCSI classes.

Refactor PureISCSIDriver into base and iSCSI classes.

This adds in a new class called PureBaseVolumeDriver that contains
all of the generic shared methods that utilize the management REST API.

The PureISCSIDriver now inherits from the base driver and implements
the functionality required for iSCSI hosts/initiators.

As part of this we’ve cleaned up some of the logging where it would
previously have hard coded the drivers class name in it, and moved some
static non-class helper methods into the base class.

These changes will ease future work to add additional drivers or split
them into separate files.

Implements: blueprint pure-base-driver
Change-Id: Iaf3979e5cccc150d09ca00daa3d9c3471f9795a0