Use custom Backend domain, refactoring to Executors responsible for calling HTTP endpoints

[lbajsarowicz]

Description

Changes introduced to HTTP request executors to support custom Hosts. Fundamental goal was to support custom Backend domain. During the development I realised that Executors need refactor according to the static variables used.

Fixed Issues

1. MFTF does not support custom Backend domain.
2. Remove static variable use for carrying hostname
3. MFTF did not handle REST API url with HTTPS

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

[magento-cicd2]

All committers have signed the CLA.

[coveralls]

Coverage increased (+0.008%) to 57.054% when pulling 5d6ffcb on M2Coach:custom-domain-backend into bf2143d on magento:develop.

[okolesnyk]

@lbajsarowicz Looks good to me, but to be sure that this will not brake existing tests we need to run all existing tests.