Convert server tests to httpretty

Change-Id: If18980fbe94cee389b7dab9925fdad8f920f9885
blueprint: httpretty-testing
Revert "Reuse exceptions from Oslo"

This reverts commit 3bde9c3f427110dae3480644752f49b79ac66ec5.

This change is a part of a series that is not backwards compatible.
It broke applications using the client library.

Change-Id: Id01a0a0feb601539d3855f445ebaf6a3cc000610
Partial-Bug: #1340596
Merge "Revert "Reuse exceptions from Oslo""
Merge "Convert server tests to httpretty"