fix(SAML): avoid session recreation loosing SAML context (#700)

* avoid creating the HTTP session in case of a SAML authentication as informations stored in it by keycloak are lost
This is not a big issue for most use cases as, once you get a bonita session, we don't go through keycloak filter code. However for some features like global logout, this may be an issue if you use POST response binding.
Also, it appears that Keycloak already provide a mechanism to recreate the session when logging in with the option turnOffChangeSessionIdOnLogin="false"
* update keycloak conf example

Cover [RUNTIME-293](https://bonitasoft.atlassian.net/browse/RUNTIME-293)