Migrate the preprov creds module to tempest lib

This commit migrates the preprovisioned credentials provider from
tempest/common into tempest/lib. As part of this 2 dependencies are
also migrated to lib: the InvalidTestResource exception class and the
tempest.common.fixed_network module.

Change-Id: Ib9e909aaaf81f7cb57e2382cec44598e918747b8
diff --git a/doc/source/library/credential_providers.rst b/doc/source/library/credential_providers.rst
index fea20bf..6e14296 100644
--- a/doc/source/library/credential_providers.rst
+++ b/doc/source/library/credential_providers.rst
@@ -14,3 +14,10 @@
 
 .. automodule:: tempest.lib.common.dynamic_creds
    :members:
+
+--------------------------------------
+The pre-provisioned credentials module
+--------------------------------------
+
+.. automodule:: tempest.lib.common.preprov_creds
+   :members: