feedback.client.Client.initialize(): Make host argument optional.

A host object is only used when running server-side tests. Since the
feedback client is not limited to server-side testing, we make this
argument optional and leave it to the implementation to validate that
it's been provided.

BUG=b:26509986
TEST=None

Change-Id: I87ab9d9b5d783bec1c284399a3f390a47833443d
Reviewed-on: https://chromium-review.googlesource.com/321455
Commit-Ready: Gilad Arnold <garnold@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
Reviewed-by: Dan Shi <dshi@google.com>
diff --git a/client/common_lib/feedback/client.py b/client/common_lib/feedback/client.py
index 19e8240..4557140 100644
--- a/client/common_lib/feedback/client.py
+++ b/client/common_lib/feedback/client.py
@@ -57,13 +57,14 @@
         self.finalize()
 
 
-    def initialize(self, test, host):
+    def initialize(self, test, host=None):
         """Initializes the feedback object.
 
         This method should be called once prior to any other call.
 
         @param test: An object representing the test case.
-        @param host: An object representing the DUT.
+        @param host: An object representing the DUT; required for server-side
+                     tests.
 
         @raise TestError: There was an error during initialization.
         """