MQE-1040: MFTF standalone commands do not pass down constants correctly

[aljcalandra]

Adjusted bootstrap files to allow alternative Magento installation locations relative to MFTF

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/verification tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)
• Changes to Framework doesn't have backward incompatible changes for tests or have related Pull Request with fixes to tests

[coveralls]

Coverage remained the same at 57.191% when pulling 38295a4 on MQE-1040 into b4a27e5 on develop.

[KevinBKozan]

Changes seem to work well.
There is a concern over MAGENTO_BP being set the same as PROJECT_BP in the normal bootstrap, but that was not introduced as part of your change.