[SE-2599] Fix flaky integration tests

Created by: fferrin

This PR increases the retries for spawning new OpenedX appservers by 2, to avoid some integration test failures in CircleCI.

JIRA tickets: SE-2599

Author notes and concerns:

1. This is the first approach to solve this issue given by @lgp171188.
2. Not sure how to test it given the flaky nature of the failures. Maybe leave it open for a month or something like that to see if it happens again?

Reviewers

• @mtyaka