Fix install_get_pip in order to work behind a proxy

Proxy envvars are not passed to pip when sudo, we need to export
them.

Change-Id: I67622f5ea8ecb948006e032bdc395ecf36914146
Closes-Bug: #1258155
Merge "Fix install_get_pip in order to work behind a proxy"
Merge branch 'master' of github.com:openstack-dev/devstack