cachethinpoolname is required instead of deprecated

[aRestless]

The docs for gluster_infra_cache_vars state:

cachethinpoolname - (deprecated, see: cachetarget) The existing thinpool on the volume group mentioned above.
cachetarget - The target thinpool or thick LV that should be cached

However,

gluster-ansible-infra/roles/backend_setup/tasks/cache_setup.yml

Line 21 in 3b40729

thinpool: "{{ item.cachethinpoolname }}"

has a hard dependency on that variable and cachetarget does not exist outside of the docs. It is therefore not possible to create a cache for a thicklv.