build/packages: improve pkg listing

When building the role.packages with packages list, use a standard
output to be able to parse it from external tools.
The use-case: check the upgrades / downgrades between releases.

Example of output on RH / CentOS:
libcurl 7.29.0
libasyncns 0.8
perl-Net-LibIDN 0.12
dbus 1.6.12
fence-agents-common 4.0.2

Example of output on Debian / Ubuntu:
xtrans-dev  1.3.2-1
xul-ext-ubufox  2.9-0ubuntu0.14.04.1
xul-ext-unity 3.0.0+14.04.20140416-0ubuntu1
xul-ext-webaccounts 0.5-0ubuntu

Output syntax is: <PACKAGE_NAME> <VERSION>