Remove Nova v3 API code from RestClient

Nova v3 API has disappeared, and Tempest isn't testing the API on the
gate now. In addition, Nova team is creating a new REST API "Nova v2.1
API + microversions" and the interfaces are different from v3 API.
So it is not necessary to keep Nova v3 API tests in Tempest.
This patch removes Nova v3 API code from RestClient for code cleanup.

Change-Id: I46631092e1c851f4603035c3dcc11287b777323c
Merge "Remove Nova v3 API code from RestClient"