Make Octavia devstack pass env settings to sudo pip install
Make Octavia devstack pass env settings to sudo pip install

Current code to install diskimage-builder requirements in
devstack does not pass on enviroment settings to sudo, which can
cause the pip install to fail behind a proxy.

This happens when the user has http_proxy/https_proxy values set
but not for root.  The stack.sh runs and installs for a while,
before failing on the Octavia diskimage-builder.

Change-Id: I196aca7004ea0eed713877e54ba10dcfe1d343e7
Closes-Bug: #1526108

openstack/octavia,openstack/octavia,openstack/octavia