spread.yaml: rename host's http proxy env vars

[fgimenez]

This way we make sure that we only use the host's proxy settings when we intend to.

[zyga]

Looks good, LGTM assuming this does not affect our ability to run in autopkgtests

[fgimenez]

@zyga thank you! :)

I don't think this will affect autopkgtests (just take a look at the results of the build in this PR), we use http_proxy (lower case) in autopkgtest preparation here https://github.com/snapcore/snapd/blob/master/packaging/ubuntu-16.04/tests/integrationtests#L7 setting it up for both snapd and the general environment before launching spread, and that env var is set in the autopkgtest executor (you can check it out by searching for http_proxy here https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-artful-snappy-dev-image/artful/amd64/s/snapd/20170512_165014_e079f@/log.gz), so the spread env vars are not involved in this case.