list command: can now filter results by supported platform

Added -p option to the list command. This alows filtering results by
supported platforms, e.g.

	wa list workloads -p linux

Also adding missing supported_platforms attribute to various extensions.
If an extension does not have this attribute, the assumption is that it
is supported by all available platforms.

wlauto/workloads/power_loadtest/__init__.py

@@ -42,6 +42,7 @@ class PowerLoadtest(Workload):
     See: https://www.chromium.org/chromium-os/testing/power-testing
 
     '''
+    supported_platforms = ['chromeos']
 
     parameters = [
         Parameter('board', default=os.getenv('BOARD'),