Agent paritions, formats, and mounts ephemeral disk

Signed-off-by: Damien Le Berrigaud <damien@pivotallabs.com>
Moved GetDeviceSizeInBlocks from platform to partitioner

Signed-off-by: Damien Le Berrigaud <damien@pivotallabs.com>
Extracted ubuntu stats functions to separate file

Signed-off-by: Damien Le Berrigaud <damien@pivotallabs.com>
Made disk partitioner idem potent

Signed-off-by: Damien Le Berrigaud <damien@pivotallabs.com>
Introduce idempotent disk formatter

Signed-off-by: Damien Le Berrigaud <damien@pivotallabs.com>