Use constant for microversions header name (cleanup)

Define and use a constant for the microversion header
name rather than repeat the string literal in multiple places.

Partially implements blueprint api-microversions

Change-Id: Ief296211d5cff8b3ccfc8231cb2336582bd47ef1
extract RPC setup into a fixture

This extracts the common RPC setup into a fixture. Eventually we need
to get this fixture off the base setup as < 5% of tests actually use
the RPC infrastructure today (probably more than half of those are
using it by accident due to insufficient mocking), and only 3 cells
tests actually need the exmods setting.

This moves the ball forward slightly towards getting there.

part of bp:functional-tests-for-nova

Change-Id: I7ab9f7f0615c137025aa07e4113e0d207d36a4c5
Merge "Use constant for microversions header name (cleanup)"
Merge "extract RPC setup into a fixture"