Support for distro release information

Nova has support for an /etc/nova/release file which, in Red Hat packaging, contains e.g.

 [Nova]
 vendor = Red Hat Inc.
 product = OpenStack Nova
 package = 1.el6

This was introduced by https://review.openstack.org/18082

It would make sense for all projects to use this

One place this could live is pbr.

The only sticking point is we use ConfigOpts.find_file() to locate the release file. The main reason for this is because cfg knows the project name, so it knows to look in /etc/${project} ... but pbr knows this too.

However, if you do e.g. --config-file /tmp/nova.conf it will also look in /tmp. It's this latter behaviour we'd lose by not using cfg, but it doesn't seem essential either.