Re-revert "libvirt: add version cap tied to gate CI testing"

In the following commit the 'version_cap' concept was
reverted in driver.py:

  commit 5dea55785a52b7edb383cf61fab2f65f2ae79a72
  Author: Sean Dague <sean@dague.net>
  Date:   Wed Jul 30 18:57:34 2014 +0000

    Revert "libvirt: add version cap tied to gate CI testing"

Unfortunately, due to a rebase error, when the version checking
code was moved into host.py, the version_cap was re-introduced:

  commit 069ad9a5b4b2cc9b49b21b09b0667909886961cf
  Author: Daniel P. Berrange <berrange@redhat.com>
  Date:   Fri Jul 4 15:12:38 2014 +0100

    libvirt: introduce new 'Host' class to manage the connection

This change reverts it once more

Change-Id: Icf4b2fff1c87d006c9827e5d27b1124e24255c0a
Bump major version of Scheduler RPC API to 4.0

Since Juno, the below deprecated methods were removed from the RPC client:
 - schedule_run_instance
 - prep_resize

Now that we're in Kilo, we only need to care about changes in between 3.0 and
4.0, and there is only one backwards compatibility to leave about flavors.

NOTE : instance_uuids can now be removed from request_spec but it will be
done in a separate commit.

Partially-Implements blueprint request-spec-object

Change-Id: I42d85407f5a98c22607536446f2defec6a994cbc
Remove duplicated constant DISK_TYPE_THIN

Change-Id: Id6d3501e3c8b8cbd392fb533bcfcae288224dcc4
Merge "Re-revert "libvirt: add version cap tied to gate CI testing""
Merge "Remove duplicated constant DISK_TYPE_THIN"
Merge "Bump major version of Scheduler RPC API to 4.0"