Make importing of "local dependencies" consistent

[Jay Pipes]

This bug was spurred by a conversation resulting from questions arising from https://review.openstack.org/#/c/80741/:

http://paste.openstack.org/show/73678/

There are a number of places in Nova where a submodule depends on an external library, but that external library is not (for various reasons) in the global requirements file. Examples of these kind of external "local dependencies" include:

* libvirt (used in nova.virt.libvirt)
* guestfs (used in nova.virt.disk.vfs.guestfs)
* evzookeeper, zookeeper, and evzookeeper.membership
* iboot (nova.virt.baremetal)

We should develop some documentation (in HACKING?) that discusses the appropriate way to import these "local dependencies", and then ensure each one in above list is done consistently.

[Sean Dague (sdague)]

I am adding this to the OpenStack CI project because what I think this is about is common infrastructure for optional dependencies. Which I think we're discussing now. This will become more important for functional testing down the road.