Add automatic creation and deletion of Purity hosts for PureISCSIDriver

Add automatic creation and deletion of Purity hosts for PureISCSIDriver

The driver will now be responsible to manage hosts for new initiators
which do not already have a host created for them. This will allow for
backwards compatibility with the previous version that relied on hosts
being pre-configured for use by Cinder.

Implements: blueprint pure-iscsi-automatic-host-creation
Change-Id: I04b554e1a21128d55eddc21afda7108a868e4248