Clean up the python prepare_binary script:
Clean up the python prepare_binary script:

- Specify python2.7 when extracting options from pants.ini during thrift bootstrap.
- We must run pants from the repo root.

Reviewed at https://reviews.apache.org/r/54550/

medallia/aurora,rdelval/aurora,apache/aurora,apache/aurora,thinker0/aurora,crashlytics/aurora,apache/aurora,rdelval/aurora,thinker0/aurora,rdelval/aurora,thinker0/aurora,apache/aurora,medallia/aurora,crashlytics/aurora,rdelval/aurora,medallia/aurora,crashlytics/aurora,medallia/aurora,medallia/aurora,apache/aurora,thinker0/aurora,crashlytics/aurora,thinker0/aurora,rdelval/aurora,crashlytics/aurora,thinker0/aurora,rdelval/aurora,medallia/aurora,apache/aurora,crashlytics/aurora