Closes#224
* feat(provider): adds plain registry content property
* docs(provider): new plain config property
* chore: comments why the order of auth parsing should not be changed
* Set up test infrastructure with local registry and custom images.
* Updated travis docker version and usage of new test infra.
* Made acc tests constantly output test results.
* Tmp acc test files are ignored.
* Fixed tests with new infra.
* Allowing insecure registries for acc tests. Added fallback for v1 registries.
* Added private image cleanup after tests.
* Refined acc test structure to confirm tf provider standards with make testacc.