Modify default prefix for solidfire account

Modify default prefix for solidfire account

The SolidFire driver creates a unique account at volume
creation composed of <cinderhost-tenantID>.This is fine
until one tries to implement HA Cinder or moves their
Cinder node to another machine.

This patch changes the default setting to be an empty string
(ie no prefix), but still allows backward compatability by setting
the config option to 'hostname'.

DocImpact: Changes default for account-prefix, for backwards compatability
use the non-default 'hostname'

Change-Id: I500ea14c1b4e9e3efd51c035cc3c4e6232da9dd6
Closes-Bug: 1265208