feat(storage): config support to delete partitions

[joseivanlopez]

Allow using delete and deleteIfNeeded for a partition section in the storage JSON config, according to the auto_storage document.

Notes about the schema:

• delete and deleteIfNeeded cannot be set at the same time.
• If delete is set, then search is mandatory and no other property is accepted.
• If deleteIfNeeded is set, then search is mandatory and size is optional. No other property is accepted.

Examples:

• Delete a partition:

{
  "search": "/dev/vda1",
  "delete": true
}

• Delete a partition on demand:

{
  "search": "/dev/vda1",
  "deleteIfNeeded": true
}

• Resize or delete a partition on demand (resize is not implemenetd yet):

{
  "search": "/dev/vda1",
  "size": { "min": 0 },
  "deleteIfNeeded": true
}

[coveralls]

coverage: 71.757%. remained the same
when pulling 607b251 on joseivanlopez:storage-delete
into 615f22e on openSUSE:master.