Start of Tempest external test plugin support

This tag signifies the start of support in Tempest for using the
external test plugin interface. The interface is considered stable
starting with this tag. Refer to the in-tree documentation for more
information about the interface.

The releases supported after this tag haven't changed since the last
tag, they are:
  * Juno
  * Kilo

The release under current development as of this tag is Liberty,
meaning that every Tempest commit is also tested against master during
the liberty cycle. However, this does not necessarily mean that using
Tempest as of this tag will work against a Liberty (or future release)
cloud.
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