Abstract creation of cache device into a single method
Abstract creation of cache device into a single method

The action varies a bit depending on whether the cache device is new or not,
but the benefit of the abstraction is still pretty clear.

Signed-off-by: mulhern <7b51bcf507bcd7afb72bf8663752c0ddbeb517f6@redhat.com>

mulkieran/stratisd,stratis-storage/stratisd,stratis-storage/stratisd,stratis-storage/stratisd,trgill/stratisd,mulkieran/stratisd,trgill/stratisd