packages: ignore yum check-update returned code

“yum check-update” execution code is not really useful, we ignore it for
the moment.

Retcode:
 - 0: success
 - 100: success with update packages to install
 - 0: RHN communication failure ( https://access.redhat.com/site/solutions/72573 )
openstack: add an empty “exclude” file
opentstack-compute: fix the package name on CentOS
minor: indent code

replace \t with 4 spaces
makefile: use simplified targets

use the “foo” alias target instead of the “$(INST)/foo.done”. This
remove a bit of complexity for the same result.
openstack-compute: packages is an associative array

declare packages as an associative array otherwise the ${foo["key"]}
syntax won't work and only returns the last last value (rpm in this
case)
minor: drop a double space in a message string
init: avoid the dependency on bash

sh -c "foo" is available in POSIX sh like dash. There is no need
to depend on bash directly.
Merge remote-tracking branch 'goneri/master'