config/v2_1: copy in v2_0 to v2_1 translator
config/v2_1: copy in v2_0 to v2_1 translator

Copy in the translator that goes from 2.0.0 to 2.1.0 from
config/translate.go, since this package will now be expected to accept
older configs than 2.1.0.

ajeddeloh/ignition,coreos/ignition,ajeddeloh/ignition,ajeddeloh/ignition,coreos/ignition,coreos/ignition