deploy: Check permissions after installing pkgs

Currently archipelago runs as archipelago:archipelago. Pithos and
Cyclades depend on archipelago and thus gunicorn must run as
www-data:archipelago. There is a known bug in gunicorn that does not
set supplementary groups when changing uid [1]. Therefore all conf
files under /etc/synnefo must be root:archipelago and not
root:www-data.

Since the snf-* packages are shipped with root:www-data conf files
we must chown them after installation. This patch, as a workaround,
checks if the user archipelago exists and acts accordingly.

Note that on a multinode setup, only Pithos and Cyclades
need those permissions. On a single node setup all components that
add files under /etc/synnefo or run via gunicorn should check
permissions during setup.

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=762448

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>