Add "x-compute-request-id" to a response header

The commit I5e370fd3de5ee2f8a8d13553015d88910ff5ea87 changed
"x-compute-request-id" to "x-openstack-request-id" on v2.1 API.
But this change is backwards incompatible against v2.0 API.

This patch makes all v2.1 APIs return "x-compute-request-id" in
the header.

Partially implements blueprint v2-on-v3-api

Change-Id: Ibe46e4004a7124ddc9a622fa7f26b9ec10a9a702
Add API json schema for server_external_event(v2.1)

Partially implements blueprint v2-on-v3-api
Change-Id: I1c6c7e958a6d01a58c9dcbc17281bd7ee9768588
Merge "Add API json schema for server_external_event(v2.1)"
Merge "Add "x-compute-request-id" to a response header"