add the openstack-compute target
simplify a bit the openstack-compute.install file
get_mirror(): do not block if distro is unknown

get_mirror() is evaluated at every run without checking $DIST.
On RHEL/CentOS, we don't use the get_mirror() result because yum
already come with it own mirror detection mechanism.

That's the reason why the get_mirror() call on RHEL/CentOS should
fail but not break the script execution.
use a stamp for every target and .PHONY

This way make know the result is already up to date and won't
try to construct the target.

"make" uses the date of the stamp file to known when the last
build of the target has been done.

With this changes, make will check this date before building the
target and so avoid the execution of the same target two time in
a row.

A .PHONY target is one that is not really the name of a file; rather it
is just a name for a recipe to be executed when you make an explicit
request. There are two reasons to use a phony target: to avoid a
conflict with a file of the same name, and to improve performance, see:
http://www.gnu.org/savannah-checkouts/gnu/make/manual/html_node/Phony-Targets.html
remove_packages() first param is the chroot dir
ensure yum is not interactive
avoid the use of global variable with no reason