The wlsHome property of the WebLogic containers should default to a pre-defined environment variable

The `wlsHome` property does not have a default value at present, and users are expected to specify this in arquillian.xml; a system property could be used in arquillian.xml, but this would require Surefire or the test-execution environment to pass an existing environment variable as a system property.

It would useful to default this to the `WL_HOME` environment variable to preserve the intent of the property. However, we could consider using the `WLS_HOME` environment variable (this points to the `WL_HOME/server` directory) since all usages refer to resources in the server sub-directory.