ui: OS icons for scientificlinux and coreos

Fixes grnet/synnefo#321
Closes grnet/synnefo#326
pithos: Add node_container_usage function

Calculate container usage for a specific path.
pithos: Directly compute usage on container reassign

Compute the container usage (needed for the reassign commission)
rather than depending on statistics.
admin: Remove redundant resource check

When checking if a resource is useful, do not check the resource.uplimit
value since it doesn't provide any concrete information.

Fix apyrgio/synnefo#307
branding: Add cloudbar icon & admin branding fixes
astakos: Remove unused ASTAKOS_SITENAME setting

ASTAKOS_SITENAME is deprecated in favor of BRANDING_SERVICE_NAME
admin: Remove unused setting from template

Remove BRANDING_ADMIN_LOGO_URL setting from admin home template