Let the test client serialize request's body

The functional test's client serializes each request content before
sending it through the wire. The _post_large_bulk_insert used to do
string formatting on a already serialized list, which ended up in a
double serialization, hence the request exceeded the max size.

The patch changes the behavior mentioned above in favor of using plain
python objects and letting the client do the serialization step.

Change-Id: If67422ea48eddfd9d35f06c68bac6a3e417fe214
Closes-bug: #1383604
Merge "Let the test client serialize request's body"