Add hypervisor_type option

In general, Tempest sends a request to Nova without any specific host
in most tests for booting a server, and Nova selects a valid host.
However some tests specify a host and this option is a hint to select
the host on heterogeneous compute environment.

Closes-Bug: #1611683

Change-Id: Ic15de7be1000c82bc13517f0bc5cf20d5248873b
2 files changed