e2e-tests: Replace hardcoded scheme with property

Replace the previously hardcoded scheme for http requests with the new
http_scheme property. The core framework documentation [1] now includes
how it can be used to (e.g.) switch test scenarios from http to https.

[1] https://gerrit-review.googlesource.com/c/gerrit/+/275453/1/Documentation/dev-e2e-tests.txt

Change-Id: I02a0f6459fa9f8b311f9035b2673732382c33ffa