Added --yes to final cache creation.

[mnlipp]

Experimenting with caching, I found "create" to be hanging. When you create a new cache volume and reuse space that has previously been used as cache (I had executed a remove immediately before the create), lvconvert prompts with:
Do you want wipe existing metadata of cache pool test/test_cache? [y/n]:
which causes the execution to hang.

As lvcache has created volume test_cache in the steps leading to the final convert, any existing metadata can savely be wiped.