Remove tests for Nova V3 API os-simple-tenant-usage

Removes the positive and negative tests for the Nova V3 API
extension os-simple-tenant-usage as the extension will not
be supported in the final Nova V3 API release.

See mailing list thread here:

http://lists.openstack.org/pipermail/openstack-dev/2014-February/026670.html

and the Nova corresponding patch:

https://review.openstack.org/#/c/72058/

Change-Id: Ia046f1d28dbf97804fc472ae2ec0f77e6e50335d
Remove using of deprecated self.headers (part1)

Attr self.headers in rest client is deprecated now, and
should be removed from usage by service clients.

This patch removes first bunch of attrs.
Second and last part is intended to be in another patch,
with removing from all other service clients
and from rest client itself.

Change-Id: I28981738a9e7f39343cf530c2642315f0a5b22f2
Partially-implements: bp refactor-rest-client
Merge "Remove tests for Nova V3 API os-simple-tenant-usage"
Merge "Remove using of deprecated self.headers (part1)"