IMPALA-3774: fix download_requirements for older Python versions
IMPALA-3774: fix download_requirements for older Python versions

Pip always runs the setup.py file in downloaded tarballs to get
metadata. Impyla's setup.py does not work in some older python
installations since find_packages() in setuptools does not support
the 'include' argument.

As a workaround, use our pip_download.py script to download Impyla
instead of pip.

Testing:
Confirmed that Jenkins build successfully downloaded the pip packages
and was able to bootstrap the virtualenv.

Change-Id: Id8801493c0f4caab2273383333ffbe2729b8339b
Reviewed-on: http://gerrit.cloudera.org:8080/3574
Reviewed-by: Tim Armstrong <2f8e34250f03a653da3dc27f7ec203eafdde80fc@cloudera.com>
Tested-by: Tim Armstrong <2f8e34250f03a653da3dc27f7ec203eafdde80fc@cloudera.com>

michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala