[BugFix] sign_alg/digest_alg policy config patched

[peppelinux]

This PR fixes: #212

Now in configuration of both backend and fronted we can use both W3C format or saml2.xmldsig attr name like follows:

            # they works both
            # sign_alg: 'SIG_RSA_SHA384'
            # digest_alg: 'DIGEST_SHA384'
            sign_alg: "http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"
            digest_alg: "http://www.w3.org/2001/04/xmlenc#sha256"

The merge would be from this specialized branch created for this patch, see diffs for better view.
"Unfortunately" this code is too much coupled with my pplnx-dev branch and in this PR wll also came a support for selectable sign and digest algs for satosa backeds, already purposed here:
#214

Hope you like and also you need it as I needed it when I developed it

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[peppelinux]

I there any possibility to discuss this patch together and, after a good sematic refactor, merge it to the master branch? This PR is just a bugfix

[peppelinux]

@c00kiemon5ter is there any revisions on this PR?
Probably the funcion named "xmldsig_validate_w3c_format" can be modified and some other things, let me know

[peppelinux]

@c00kiemon5ter it's just a bugfix, I can refactor this as well if you start a revision.
This could be coupled with https://github.com/IdentityPython/SATOSA/pull/214/files

[c00kiemon5ter]

Also closing this. This should be handled by pysaml2 and the signing_algorithm and digest_algorithm options.

The frontend used the sign_alg and digest_alg options and we should now remove them.

---

As I noted on #214 (comment) YAML and satosa are not responsible to resolve symbols.

            # they works both
            # sign_alg: 'SIG_RSA_SHA384'   # NOT OK
            # digest_alg: 'DIGEST_SHA384'  # NOT OK
            sign_alg: "http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"  # OK but now with signing_algorithm
            digest_alg: "http://www.w3.org/2001/04/xmlenc#sha256"          # OK but now with digest_algorithm