More intuitive env var names for global-scope options.

Previously for global option --bar-baz you had to set PANTS_DEFAULT_BAR_BAZ.
Now we allow PANTS_BAR_BAZ, eliding the explicit global scope 'DEFAULT'.

If the option name itself already starts with 'pants-', e.g., --pants-workdir,
we also allow PANTS_WORKDIR, instead of PANTS_PANTS_WORKDIR or
PANTS_DEFAULT_PANTS_WORKDIR.

Using env vars to set option values isn't necessarily common, but this will make
it more conveninent and intuitive. Also, it will allow the PANTS_CONFIG_OVERRIDE
mechanism to be retconned into a --config-override option.

Testing Done:
CI passes.

Reviewed at https://rbcommons.com/s/twitter/r/1441/
Sync of https://github.com/pantsbuild/pants/commit/217a051cee572bd255d166fa0a228b8f33f3ae4b