chromeos_test_image: only set default password if keyfile was not specified

wlauto/devices/linux/chromeos_test_image/__init__.py

@@ -42,12 +42,15 @@ class ChromeOsDevice(LinuxDevice):
         Parameter('core_names', default=[], override=True),
         Parameter('core_clusters', default=[], override=True),
         Parameter('username', default='root', override=True),
-        Parameter('password', default='test0000', override=True),
         Parameter('password_prompt', default='Password:', override=True),
         Parameter('binaries_directory', default='/usr/local/bin', override=True),
         Parameter('working_directory', default='/home/root/wa-working', override=True),
     ]
 
+    def validate(self):
+        if self.password is None and not self.keyfile:
+            self.password = 'test0000'
+
     def initialize(self, context, *args, **kwargs):
         if self.busybox == 'busybox':
             self.logger.debug('Busybox already installed on the device: replacing with wa version')