Change legacy amulet proxy env vars to fit OS_ scheme

[Ryan Beisner]

Example:
Attempted removal of AMULET_* in https://review.opendev.org/#/c/678676/1/tox.ini is causing this failure.

We can land the following changes which add OS_TEST_HTTP_PROXY and leave AMULET_HTTP_PROXY as a transitional duplicate env var to enable a transition here.

https://code.launchpad.net/~1chb1n/ubuntu-openstack-ci/os-proxy-env-vars/+merge/372016

https://github.com/juju/charm-helpers/pull/369

https://github.com/openstack-charmers/zaza-openstack-tests/pull/42

The caveat is, for legacy non-reactive charms (ie. cinder, glance et al), a charm-helper sync (`make sync`) will need to accompany the tox AMULET_ removal.

[Ryan Beisner (1chb1n)]

For legacy charms, the tox.ini may need to have OS_* envs allowed in order to run this through.

[Chris MacNaughton (chris.macnaughton)]

Additionally, the stable charms will continue to need the AMULET_* variables for a cycle ;-)