Use nova bindir of /usr/bin on Fedora

Fedora (and RHEL) like to install things in /usr/bin, e.g.

 01:30:42 Creating /usr/lib/python2.7/site-packages/nova.egg-link (link to .)
 01:30:42 Adding nova 2013.2.a1031.gfc5137d to easy-install.pth file
 01:30:42 Installing nova-dhcpbridge script to /usr/bin
 ...

The default nova/paths.py defaults to /usr/local/... which causes
quite a few errors when stressing various parts of nova and tools
aren't found.

Change-Id: Iaa93af96ddfb4deb6d16ea1dcac25ae6ed1e317d