Code review comment for lp:~raharper/curtin/set_bcache_cache_mode

I've addressed two things:

1) Blake highlighted that relying the the bcache storage config id isn't robust; it could be any string though in practice it tends to the the correct value. Instead, use the backing device to look up the bcache device name via sysfs holders dir.

2) Switch testcase to use check_file_regex