Rename for clarity

Signed-off-by: Tony Hansmann <thansmann@pivotallabs.com>
Add DiskImage#while_mounted

  * ensures that image is unmounted / unmapped

Signed-off-by: Tony Hansmann <thansmann@pivotallabs.com>
DiskImage uses a tmpdir mount if none is specified

Signed-off-by: Tony Hansmann <thansmann@pivotallabs.com>
Make DiskImage less noisy

Signed-off-by: Tony Hansmann <thansmann@pivotallabs.com>
Stemcell specs handle disk image mounting

Signed-off-by: Tony Hansmann <thansmann@pivotallabs.com>