Moved snapd installation sooner in the installer

This changes moves the check of snapd package sooner in the installer,
and installs it if missing. It also modifies check_packages function
so that the output is not redirected to /dev/null. Finally, some echos
were added to know the progress of the check.

Change-Id: I3f09af72542ae7e58d923155d092abb44f292c44
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>