Change message confirmation class of Openshift discovery of image registry URL

When handling confirmation dialog of discovery feature in openshift connection wizard, MessageBox is used. This class is subclass of org.eclipse.swt.widgets.Dialog which is mentioned to be using built-in platform dialogs that are not accessible via RedDeer framework or any other way. I cannot handle this kind of dialog in integration tests.

Thus, I would like to change the MessageBox implementation to MessageDialog which is from jface layer and is easily obtainable via RedDeer.

To advocate this patch, MessageDialog is commonly used in openshift plugin while MessageBox is not (just this one occurrence).