Fix NovaEnableRbdBackend to be role specific
Fix NovaEnableRbdBackend to be role specific

This [1] change enabled to have role specific NovaEnableRbdBackend
parameter, however global parameter were allways overriding role
specific hiera after it was set via RoleParametersValue

[1]: I9f40a2a3561fcb1d1fec9d9c3c1f9cabaf02650c

Change-Id: Iab58223a6140e42028d4d2a7305c05fd7ae3a9f0

openstack/tripleo-heat-templates,openstack/tripleo-heat-templates