deploy: Check permissions after installing snf-*

Currently archipelago runs as archipelago user. Pithos and Cyclades
depend on archipelago and thus gunicorn must run as
www-data:archipelago. Therefore all conf files under /etc/synnefo
must be root:archipelago.

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.

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