Migrate the dynamic creds module to tempest lib

This commit migrates the dynamic credentials provider from
tempest/common into tempest/lib.

Change-Id: Id8d60e4d7c22bc72c3e48bc768509ff0cc0c89d5
diff --git a/doc/source/library.rst b/doc/source/library.rst
index 29248d1..a461a0f 100644
--- a/doc/source/library.rst
+++ b/doc/source/library.rst
@@ -68,3 +68,4 @@
    library/api_microversion_testing
    library/auth
    library/clients
+   library/credential_providers