docs: Add information on how to instantiate a HostVirtual driver.
Re-generate documentation fixtures.
Add allow_insecure=True attribute to the HostVirtual connection class.
Update docstrings.
Update HostVirtual compute driver to use super and take in all the supported
arguments.
Update HostVirtual DNS driver constructor to only take in "key" and other
supported arguments. Previously it also took "secret" which it silently
ignored.

Part of LIBCLOUD-483.
Update CHANGES.