introduced shared.url.scheme in configuration, see #747

By default ```shared.url.scheme``` is http (see config/shared.maven.filters), which will match most of the setups.

People having an SDI configured for full HTTPS will take advantage of the newly introduced ```shared.url.scheme``` variable.

Note: this setting is only relevant for the SDI external URL.
For now, we always assume that it internally uses HTTP.
will have no consequence on SDIs accessed through http, but will prevent mixed content warning in some browsers when accessed through https.