[2.5] Can create a bcache from a pre-existing bcache.

[Newell Jensen]

While playing around with bcaches I noticed that in the UI, you can create a bcache from a pre-existing bcache. To reproduce, create a bcache. Once the bcache is created it will show up in the list of available disks and partitions. Click on it and choose to create a bcache again. After this, when trying to deploy the installation fails as seen below:

curtin: Installation started. (18.1-51-gb812ae80-0ubuntu2~18.04.1)
third party drivers not installed or necessary.
UUID: b84f88c9-4100-45f5-b010-6207565400a2
Set UUID: 7abffb02-0c31-4a2b-947e-8efc130feda4
version: 1
block_size: 1
data_offset: 16
UUID: be7cac36-90d5-4a9d-ae50-778def68462c
Set UUID: c239917b-a6d8-4f07-9a88-2f768b96172e
version: 1
block_size: 1
data_offset: 16
UUID: 8ca4bdcd-c339-4934-b354-ca9027a20f35
Set UUID: b34e1150-16a2-4418-a8ae-28f1ee636726
version: 1
block_size: 1
data_offset: 16
An error occured handling 'bcache4': RuntimeError - ('Unexpected old bcache device: %s', '/dev/bcache0')
('Unexpected old bcache device: %s', '/dev/bcache0')
curtin: Installation failed with exception: Unexpected error while running command.
Command: ['curtin', 'block-meta', 'custom']
Exit code: 3
Reason: -
Stdout: UUID: b84f88c9-4100-45f5-b010-6207565400a2
        Set UUID: 7abffb02-0c31-4a2b-947e-8efc130feda4
        version: 1
        block_size: 1
        data_offset: 16
        UUID: be7cac36-90d5-4a9d-ae50-778def68462c
        Set UUID: c239917b-a6d8-4f07-9a88-2f768b96172e
        version: 1
        block_size: 1
        data_offset: 16
        UUID: 8ca4bdcd-c339-4934-b354-ca9027a20f35
        Set UUID: b34e1150-16a2-4418-a8ae-28f1ee636726
        version: 1
        block_size: 1
        data_offset: 16
        An error occured handling 'bcache4': RuntimeError - ('Unexpected old bcache device: %s', '/dev/bcache0')
        ('Unexpected old bcache device: %s', '/dev/bcache0')

Stderr: ''