Add hacking test to block cross-virt driver code usage

The baremetal driver has been accessing some libvirt driver
private classes and config variables. Any code that is intended
to be shared across virt drivers should be in a common package
instead. Add a hacking file to enforce this for imports and
config options so that no further problems like this are made
in the future.

NB, this skips verification of the baremetal driver until
bugs 1261826 and 1261827 are fixed, or the baremetal driver is
removed, whichever comes first.

Change-Id: Ifbfe597795795a847830f9bd937dc459cd37d118
Closes-Bug: #1261842

HACKING.rst

@@ -13,6 +13,12 @@ Nova Specific Commandments
   This enforces a guideline defined in ``nova.openstack.common.db.sqlalchemy.session``
 - [N310] timeutils.utcnow() wrapper must be used instead of direct calls to
   datetime.datetime.utcnow() to make it easy to override its return value in tests
+- [N311] importing code from other virt drivers forbidden
+  Code that needs to be shared between virt drivers should be moved
+  into a common module
+- [N312] using config vars from other virt drivers forbidden
+  Config parameters that need to be shared between virt drivers
+  should be moved into a common module
 - [N123] vim configuration should not be kept in source files.
 
 Creating Unit Tests