Improve handling and testing of expressions used in appclient configuration

The AppClientSubsystemResourceDefinition 'file', 'deployment', 'host-url' and 'connection-properties-url' attributes all support expressions. The values that drives these come via cmd line params to the appclient Main.

1) Exercise this logic by having AbstractSimpleApplicationClientTestCase/AppClientWrapper pass expressions over the cmd line instead of the resolved values.

2) Fix the problem this revealed where the appclient Main class is checking the validity of the param that represents the file. That check is premature as the value being checked may be an expression. Defer the check until AppClientSubsystemAdd after the expression is resolved.