Use proper SENTINAL to mark complex ENV vars as enabled or disabled

[mrnicegyu11]

Is there an existing issue for this?

• I have searched the existing issues

Planned changes

Use explicit variables such as DIRECTOR_V2_POSTGRES__POSTGRES_DB=mydb instead of POSTGRES_DB=mydb:
    allows for sorting ENVs per service instead of having sub settings in the middle,
    this would allow to disable postgres only in the director-v2 by doing DIRECTOR_V2_POSTGRES__POSTGRES_DB= for example
    no need to pass DIRECTOR_V2_POSTGRES at all

This is a sub issue from ITISFoundation/osparc-issues#675 via @sanderegg

[mrnicegyu11]

Orginally this was raised in todays discussion w.r.t. ITISFoundation/osparc-ops-environments#866