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 7.29.0-19.el7
libasyncns 0.8-7.el7
perl-Net-LibIDN 0.12-15.el7
dbus 1.6.12-8.el7
fence-agents-common-4.0.2 21.el7

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>-<RELEASE>

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge pull request #111 from enovance/list_pkg

build/packages: improve pkg listing