Set default http-based exception as `HttpError`

Since exceptions from oslo is used, ClientException doesn't have any
attributes related to HTTP, so `HttpError` should be set as default
http-based exception.

Also, attribute `code` is used in several project, so it should be
returned and marked as deprecated.

Partial-Bug: #1322183
Change-Id: I3c71e2d25d6e36b5bac0f2b3add74d6747cf7c25
Convert certificate tests to httpretty

Change-Id: Ic7eacb75f90b383ecacf01269033c0a37bd7528c
blueprint: httpretty-testing
Convert Cloud Pipe tests to httpretty

Change-Id: I0a737a5c66831edafbd671bfd49017bc1274fca3
blueprint: httpretty-testing
Convert Fixed IP tests to httpretty

Change-Id: I90a14b51dba6c0b64f0a264e762a0795a2566ae6
blueprint: httpretty-testing
Merge "Set default http-based exception as `HttpError`"
Merge "Convert certificate tests to httpretty"
Merge "Convert Cloud Pipe tests to httpretty"
Merge "Convert Fixed IP tests to httpretty"