Make validation_resources a stable interface

Move the module to tempest.lib, add unit tests, release notes and
update imports.

Change-Id: I76b3b8e9a83bd83a6b5742b6d2ceb84642f653ad
diff --git a/doc/source/library.rst b/doc/source/library.rst
index a461a0f..074d642 100644
--- a/doc/source/library.rst
+++ b/doc/source/library.rst
@@ -69,3 +69,4 @@
    library/auth
    library/clients
    library/credential_providers
+   library/validation_resources