Add DiskImage for manipulation of image files

Signed-off-by: Jesse Zhang <jz@pivotallabs.com>
Run stemcell tests after building image

  * in preparation for testing against the .img instead of chroot dir

Signed-off-by: Jesse Zhang <jz@pivotallabs.com>
Stemcell specs run against generated .img

  * instead of pre-image chroot dir

Signed-off-by: Jesse Zhang <jz@pivotallabs.com>
Write basic /etc/fstab for CentOS

  * otherwise it will mount / read-only

Signed-off-by: Jesse Zhang <jz@pivotallabs.com>
Fix rubocop method-too-long issue

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