Expand documentation on HTTP_PROXY, HTTPS_PROXY, NO_PROXY

[juanvallejo]

Right now, current documentation covers proxy enrivonment variables
by mentioning that they can be set in podSpec templates, and as part of a
node's configuration. Documentation does not mention, however, that these
environment variables can also be set on the environment from which an
oc binary is being executed.

It would be good to expand docs (and even cli help output) to mention that
these environment variables are taken into account by an oc binary, and give
some use-cases on when they might be used by oc to talk to a master.

cc @gabemontero @openshift/cli-review @bparees

[openshift-bot]

Issues go stale after 90d of inactivity.

Mark the issue as fresh by commenting /remove-lifecycle stale.
Stale issues rot after an additional 30d of inactivity and eventually close.
Exclude this issue from closing by commenting /lifecycle frozen.

If this issue is safe to close now please do so with /close.

/lifecycle stale

[openshift-bot]

Stale issues rot after 30d of inactivity.

Mark the issue as fresh by commenting /remove-lifecycle rotten.
Rotten issues close after an additional 30d of inactivity.
Exclude this issue from closing by commenting /lifecycle frozen.

If this issue is safe to close now please do so with /close.

/lifecycle rotten
/remove-lifecycle stale

[bparees]

/remove-lifecycle rotten
/lifecycle frozen

[bparees]

/lifecycle frozen