Make add_member use **kwargs and doc string update

As we discussed on
http://lists.openstack.org/pipermail/openstack-dev/2015-July/068864.html
All http POST/PUT methods need to contain **kwargs as their arguments.

This patch makes add_member use **kwargs.
Also update doc string for the same.

Partially implements blueprint consistent-service-method-names

Change-Id: I67810b4e9c313611393e896f68d3b4a1dda77bea
diff --git a/tempest/services/image/v1/json/images_client.py b/tempest/services/image/v1/json/images_client.py
index 4884106..3406db8 100644
--- a/tempest/services/image/v1/json/images_client.py
+++ b/tempest/services/image/v1/json/images_client.py
@@ -265,11 +265,14 @@
         body = json.loads(body)
         return service_client.ResponseBody(resp, body)
 
-    def add_member(self, member_id, image_id, can_share=False):
+    def add_member(self, member_id, image_id, **kwargs):
+        """Add a member to an image.
+
+        Available params: see http://developer.openstack.org/
+                              api-ref-image-v1.html#addMember-v1
+        """
         url = 'v1/images/%s/members/%s' % (image_id, member_id)
-        body = None
-        if can_share:
-            body = json.dumps({'member': {'can_share': True}})
+        body = json.dumps({'member': kwargs})
         resp, __ = self.put(url, body)
         self.expected_success(204, resp.status)
         return service_client.ResponseBody(resp)