Add opportunity to directly update headers

Added change in request's methods in rest_client
for directly updating headers.
If someone want to add some custom headers to default headers
he can pass variable extra_headers=True in one of
the request's methods(POST/PUT/...) and headers should be
added to default headers

Change-Id: Ifbe49a9f7a6e2de780953ef9dcd39e443d74d151
Closes-bug: 1295661
Merge "Add opportunity to directly update headers"
Merge branch 'master' of https://github.com/openstack/tempest