Allow for ops to direct traffic to an internal appservers pool and not edge servers

• introduce another env variable WIKIBASE_REPO_HOST_ALIAS
  • has a default value of appservers.discovery.wmnet
  • application refuses to start if value is not set
• during axios setup
  • parse the value coming in from WIKIBASE_REPO
  • extract its host part (from now on referred to as HOST)
  • substitute it for the value coming in from WIKIBASE_REPO_HOST_ALIAS
  • use the HOST value as the Host header

Note:

• code created to to achieve this should be exercised in all stages (no "breaks in prod")

Is reaction to T220402