Initial support for CentOS stemcell creation

[#54319716]

Signed-off-by: Tony Hansmann <thansmann@pivotallabs.com>
Rename image_install_grub2 back to _grub

Signed-off-by: Tony Hansmann <thansmann@pivotallabs.com>
Extract GemComponent
Rearrange #component_needs_update
Move behavior down to GemComponent
Remove lazy gem building, we always rebuild all gems
Remove unused method
Move gem building to GemComponent
Explore list of dot_gems so they can eventually be promoted in parallel
Merge remote-tracking branch 'origin/master' into develop

For commit that was merge directly into master using the "merge button"
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>