Only enable nodepool precise-backports on Ubuntu

In preparation for using Fedora with this script, only try to add
the precise-backports repo if we're using Ubuntu.

Fedora 20 has jq in the regular repos, so no Fedora version is
required.

Also fix permissions on install_devstack_dependencies.sh

Change-Id: I3544ddecb0e3ba9dc863f264d67669acd06616f3