Avoid triggering the authentication popup when starting Tomcat instance

[NicolaIsotta]

The first time I start a Tomcat instance after opening netbeans, I get this annoying popup:

This PR fixes the issue by passing the correct credentials when testing the connection to the Tomcat manager.

NB: the issue happens only when the Tomcat Server header is empty, so by default on Tomcat 8.5+

fixes #8646

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[mbien]

makes sense, thanks!