app: Remember SSL credentials so that it is possible to retrieve them

Unfortunately after creating SSL context it is not possible, or at least
I could not find how, to retrieve original credentials with which the
context was created. However wendelin.core needs to be able to take a
client storage, reconstruct zurl to refer to that particular storage,
and pass that zurl to wcfs, so that wcfs, in turn, could access the same
ZODB database.

Given a NEO client instance, it is already possible to retrieve
master_nodes, cluster name, and detect whether SSL is being in use.
However without being able to retrieve original SSL credentials,
reconstructed zurl will not be full and wcfs won't be able to use
exactly the same secrets as python part does.

-> Help wendelin.core by remembering which ca/cert/key were used to
build SSL context.

This information is used by zstor_2zurl in wendelin.core here:

https://lab.nexedi.com/nexedi/wendelin.core/blob/885b3556/lib/zodb.py#L390-418

/cc @levin.zimmermann
/reviewed-by @jm
/reviewed-on !21

(cherry-picked from commit 17af7f27)

neo/lib/app.py

@@ -72,6 +72,9 @@ class BaseApplication(object):
                     " provided: the CA certificate, and the certificate"
                     " of this node with its private key.")
             ca, cert, key = ssl
+            # remember ca/cert/key, so that zstor_2zurl in wendelin.core could
+            # retrieve them and fully reconstruct neos:// url of the storage
+            self.ssl_credentials = ssl
             import ssl
             version, version_name = max((getattr(ssl, k), k)
                 for k in dir(ssl) if k.startswith("PROTOCOL_TLSv"))