Use TableauServerUser only when using integrated authentication

[kaklakariada]

Logic in connectionProperties.js that uses connectionHelper.attributeTableauServerUser is only allowed when

attr[connectionHelper.attributeAuthentication] === connectionHelper.valueAuthIntegrated;

(See documentation). The runningOnServer() call and logic needs to be conditional on useKerberos() being true.

Rational: we can only have the value of attr[connectionHelper.attributeTableauServerUser] used during Kerberos (integrated auth) scenarios. In either Desktop or Server with the Username and Password authentication the username value must always come from the value the user entered in the connection dialog: attr[connectionHelper.attributeUsername].