cnitool: deduplicate environment variable definitions

[UiP9AV6Y]

create a reusable package for environment variable to be used by cnitool and the other library code.

Signed-off-by: Gordon Bleux [email protected]

[coveralls]

Coverage decreased (-1.06%) to 71.299% when pulling fb89176 on UiP9AV6Y:feature_dedupe_env into 76aaefb on containernetworking:main.

[squeed]

This looks basically reasonable, but any of the environment variables that are cnitool-specific should probably not be in pkg. That's just because we don't want to make any public library commitments in that regard.

[UiP9AV6Y]

i have moved all cnitool specific default values back into
the cmd package to prevent having too many empty defaults
(the default has to be defined by the consumer when using
GetVal())

furthermore i have moved the CNI Args parsing logic into
the library API as this is something covered by the
SPEC

[mars1024]

Since pkg/env is supposed to be a tool package, is it better to place this constants on another package, maybe pkg/constants?