Allow re-use of bcache cache devices with separate backing devices
Decouple creation of bcache caches and backing devices by dropping
use of make-bcache -B <> -C <>, in favor of single use -B and -C
and combining backing devices with caches via sysfs attach attributes.
Update the mdadm/bcache vmtest to include a multi-bcache backing device
cached by single cachedevice test.
While validating results, fix lurking issue with mdadm.conf not getting
updated post raid configuration in target triggering LP: #964052.