Rename v3._AuthConstructor to v3.AuthConstructor

Since more auth plugins depending on v3._AuthConstructor are created
in separated modules, this class should no longer be named as it was
private.

Auth plugins using v3._AuthConstructor currently are:

- SAML2 auth plugin (under review)
- oAuth auth plugin (merged with basecode)

Change-Id: Ia097941a465a972dc7ca177a74c8fb8d21d219e6