unit speeds fix; husk burn; tanks explode small

Vehicle speeds adjusted, now 80% on clear, like tanks. Intended to coincide with speed changes here: https://github.com/OpenRA/OpenRA/pull/2722
Husks burn 10 seconds now instead of 40. More fitting for cnc.
Tanks UnitExplodeSmall now by default. 
Merge pull request #2725 from psydev/patch-5

defaults.yaml: unit speeds fix; husk burn; tanks explode small