VTOL_takeoff command always uses relative altitude

#### Issue details
When mission plan issues VTOL_takeoff command it always uses relative altitude ignoring mission planning settings for altitude reference. I can't say for sure is it MP bug or Arduplane. For me, it seems that MP's.

#### Version
MP 1.3.55
AP 3.9.0-dev

#### Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[ X ] Plane
[ ] Rover
[ ] Sub

#### Airframe type
Quadplane

#### Hardware type
SITL

#### Logs
Easily reproducible with SITL

No source file or test is named. Reproduce the VTOL_takeoff mission-plan behavior in SITL, then trace how Mission Planner applies the selected altitude reference and compare the generated command with the planning settings; done means the command honors the configured reference on Plane Quadplane missions.

Written by the indexing model from the issue text.