Add TEMPEST_ENABLE_NOVA_XML_API option

XML support of Nova API has been deprecated with the following
message:
  XML support has been deprecated and may be removed as early
  as the Juno release.
Now Kilo development cycle started, so we should disable Nova
API XML tests on Tempest.

This patch adds TEMPEST_ENABLE_NOVA_XML_API option for setting
Tempest test by devstack-gate. The default value is True because
of keeping the test coverage. After merging a devstack-gate patch
which specifes True for stable/icehouse and stable/juno, we will
set False for the master blanch.

I4acc15ce5f487738bb34a95c2261a5d05d827d8d is a Nova patch which
removes XML API support.

Change-Id: I2ab8f5c3d15b496e0b639c99bb8592533a69a265
Merge "Add TEMPEST_ENABLE_NOVA_XML_API option"