Remove using of deprecated self.headers (part2)

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

This patch removes second bunch of attrs in service clients
and from rest client itself.

part1: https://review.openstack.org/#/c/73869/

Change-Id: I109479299cd41848e78700484bed862012638c4d
Partially-implements: bp refactor-rest-client
Merge "Remove using of deprecated self.headers (part2)"