Add SYSTEM_UPDATE option for atomic hosts

[ghost]

If system_update is set to false, atomic is still being updated.

[jprovaznik]

Good catch. One nit, because [ "$SYSTEM_UPDATE" = "True" ] is checked in both branches of "if-then-else", it would be better to move "[ "$SYSTEM_UPDATE" = "True" ]" in front of the if statement, something like:

[ "$SYSTEM_UPDATE" = "True" ] || exit 0

if [ -e /run/ostree-booted ]
then
# Update the OS tree
atomic host upgrade || notify_failure "failed to run 'atomic host upgrade'"
else
# Update using traditional RPMs
retry yum install -y deltarpm || notify_failure "could not install deltarpm"
retry yum -y update || notify_failure "could not update RPMs"
fi

[ghost]

OK done :)

[jprovaznik]

Cool, also a wrong indentation sneaked in in the "if" branch, could you fix it and squash commits to one?