Add new variable to enable/disable encrypted assertions in IdP container

https://github.com/nuxeo/nuxeo-tools-docker/tree/master/shibboleth-idp-2

A new variable called ENCRYPT_SAML_ASSERTIONS will have the following possible values:

1. true (by default) which will leave the settings by default
2. false which will replace everything occurences of encryptAssertions="conditional" in the file relying-party.xml with encryptAssertions="never".
3. any other value should exit with a displayed error and exit code 1

This should be done before starting the server.

Update the README.md file accordingly.