Use run_in_chroot function instead of hand-rolling

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

  * use release files in /etc to determine OS

Signed-off-by: Tony Hansmann <thansmann@pivotallabs.com>
image_install_grub uses files in /etc to determine OS

Signed-off-by: Tony Hansmann <thansmann@pivotallabs.com>
Use Ubuntu/CentOS agnostic user/group tools

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