Add CHAP support for 3PAR iSCSI

3PAR ISCSI does not currently support CHAP. In order to implement this, volumes and hosts on the 3PAR backend will need to have CHAP credentials stored somewhere. Hosts have a CHAP credential field already available for use. The volumes will need to have CHAP credentials stored in the metadata for each volume. By storing the credentials in the volume metadata it can be accessed in OpenStack when attaching new volumes to a CHAP enabled host. When the first volume is attached to a host, CHAP credentials are set for both the volume/host. As future volumes are attached to this host the credentials can be pulled from the previously attached volume. This ensures that the CHAP credentials on new volumes are always correct.