Filter SAML2 assertion parameters with certain prefix.

Add ``assertion_prefix`` option that filters environment parameters
that will be passed to the RuleProcessor object. Parameters' names
must start with ``assertion_prefix`` value.
If not configured, ``assertion_prefix`` defaults to an empty string,
and all environment parameters are passed to the RuleProcessor.

Change-Id: I2696bbadcfff9745d8edca6c896c13fda49d636e
Closes-Bug: #1293436
Merge "Filter SAML2 assertion parameters with certain prefix."