Read arguments for thrift-linter from pants.ini.

This allows changing linter options without changing pants.

Testing Done:
./build-support/bin/ci.sh

PANTS_DEV=1 ./pants goal thrift-linter testprojects/src/thrift/com/pants/testproject:thrift-java

Bugs closed: 719

Reviewed at https://rbcommons.com/s/twitter/r/1215/
Add support for updating the environment from prep_commands.

Reviewed at https://rbcommons.com/s/twitter/r/1222/
Include traceback with failures (even without fail-fast).

Testing Done:
Ran tests.

Reviewed at https://rbcommons.com/s/twitter/r/1226/
Fix a bug where all OptionValue objects had the same underlying forwardings.

This wasn't seen earlier because in practice all tasks migrated so far had
disjoint option names.

This was exposed when I migrated two unrelated tasks that happened to have
an identically named option (--skip).

Testing Done:
CI passed.

Verified that this fixes the name collision issue that triggered its discovery in the first place.

Reviewed at https://rbcommons.com/s/twitter/r/1229/
move codegen tasks to new options

Move codegen tasks to new options

Testing Done:
I considered adding integration tests, but for these options, but the expected behavior changes in many cases consist of side effects that are non-trivial to verify (other than thrift-linter).

For "thrift-linter", I ran the "thrift-linter" goal on the various thrift-linter targets in testprojects/. I verified that I could override the "bad-thrift-strict" target using "--no-thrift-linter-strict" and get it to succeed (and vice versa with the "bad-thrift-default" and "bad-thrift-non-strict" targets.

For "gen.scrooge", I ran the "gen" goal on tasks under examples/ and testprojects/. BTW, I could not get any output from the compiler to show up ever. I verified things were working by using the "-ldebug" flag to pants and see that it was indeed adding "--verbose" to the compiler invocation.

For "gen.protoc" and "gen.thrift", I ran the "gen" goal on tasks under testprojects/. I verified that the expected flags were passed to the thrift and proto compilers using the "-ldebug" flag.

Aside: for the "gen.protoc" target, I saw a strange result when indicating "--gen-protoc-lang=python" for a target where Java was actually needed:
  Exception message: 'PythonLibrary' object has no attribute 'jar_dependencies'
But this is unrelated to my change and happens with the code on master, too:
  $ devpants goal gen testprojects/src/protobuf/com/pants/testproject/proto-ordering:protos --gen-protoc-lang=python

Reviewed at https://rbcommons.com/s/twitter/r/1203/
move ide_gen.py and friends idea_gen.py and eclipse_gen.py to new options