Fix attach_interfaces tests

Tempest's interfaces_client was not calling Nova "os-interface" API correctly.
The POST parameter interfaceAttachment was ill formed.

Change-Id: I3db0381d48919c62f3f3c6d60fc14cf4dee9c4a3
Closes-Bug: #1271642
Merge "Fix attach_interfaces tests"
Remove suffix "JSON" from Nova v3 API test classes

We have decided that XML support is removed from Nova v3 API and
the tests also has been removed from Tempest already.
Then the suffix "JSON" of Nova v3 API test classes does not have
any meanings.

This patch removes them for cleanup. This is made because of a
comment of Ie3c595fb77143e58e30404c42f358d6443a52c0b .

Part of bp:remove-v3-xml-api

Change-Id: Ic7372817777256ce723843ffd3e0bc670261aa78
Merge "Remove suffix "JSON" from Nova v3 API test classes"