Migrated microversion testing framework to tempest/lib

Tempest implements the microversion testing support and
that framework and interfaces have been tested by implementing
the Compute 2.2 microversion as well as by their unit tests.

Those should be stable interfaces so that can be consumed by other
projects microversion testing either in tempest or outside tempest.

To make them as stable interface, this commit moving that framework and
its interfaces to tempest/lib folder.

Adding release notes and library doc for those interface.

Partially implements blueprint api-microversions-testing-support

Change-Id: Icbdcfb4cd5b7fb1029eec035b9e0024be59c8d1f
diff --git a/doc/source/library.rst b/doc/source/library.rst
index 8b263f2..64bd2c2 100644
--- a/doc/source/library.rst
+++ b/doc/source/library.rst
@@ -66,3 +66,4 @@
    library/decorators
    library/rest_client
    library/utils
+   library/api_microversion_testing