Proxy support

[kivio]

Added possibility to configure proxy on VM when setup must stay behind the proxy server.

[klausw]

or FLAGS.ftp_proxy

[klausw]

FYI, review comments are intended as suggestions - we don't want the review process to turn into a barrier to contributions. Feel free to push back on requests and/or ask us to take care of issues.

[kivio]

OK, i think now it looks good

[klausw]

LGTM, thanks for making the changes. @cmccoy , any comments from your side?

[klausw]

test this please

[kivio]

OK, i was added documentation now.

In subject of test, i have that environment in my work then proxy works good, do you think about unit tests?

[cmccoy]

LGTM. There are going to be some merge conflicts due to #321. Let us know if we can help.

[klausw]

Working on the merge now - seeing some lint errors from flake8, I'll take care of them.

[klausw]

I've merged this manually to the dev branch, but this didn't work right since it erroneously applied changes to the deleted
package_managers.py file instead of the new linux_virtual_machine.py file. Working on #337 , please wait for that before continuing. I'll update this thread once that's done.

[klausw]

The dev branch is now up to date. I had combined your patches into the single change d0ebc16 with no modifications, then followed up with the two fixup changes - in case you run into merge conflicts, you could try cherry picking those separately on top of your original changes.

Also, can you please verify that things work right for you? I had slightly changed the logic as part of the move to take better advantage of inheritance, it should be equivalent but I could only do limited testing.
(I get the expected connection errors from apt when setting a nonexistent --http_proxy.)

For reference:

commit b8e2771b0d66a2d8ed2845601e9f03f097380e93
Author: Klaus Weidner <klausw@google.com>
Date:   Tue Jun 23 11:39:57 2015 -0700

    Proxy config fixup for bad merge.

    The previous proxy config patch erroneously applied changes to the deleted
    file package_managers.py, reviving it in the process. Delete it again, and
    apply its changes to linux_virtual_machine.py.

commit 6c061d2475312c88742e5c605ce6e94525c34fae
Author: Klaus Weidner <klausw@google.com>
Date:   Tue Jun 23 10:45:47 2015 -0700

    Lint fixes for proxy configuration.

    Minor doc edits, fix some over-long lines, and use the ElementTree prefix
    instead of directly importing functions into the toplevel namespace.

commit d0ebc1699b04b22dd7fab0bbe73808e5485ab8c3
Author: Marcin Karkocha <kivio@kivio.pl>
Date:   Tue Jun 23 10:41:59 2015 -0700

    Add support for using proxies in the cloud environment.

    This introduces new flags --http_proxy, --https_proxy and --ftp_proxy.
    The flag values use the same syntax as the corresponding environment variables,
    for example `--http_proxy=proxy.example.com:8080` .

    See README.md for additional information.