move to molecule 2.x

[paulfantom]

Resolves #93

• support molecule 2.x
• fix linter warnings and errors
• update guidelines on how tests work
• add travis pipeline for latest prometheus version (similar to one ran locally)
• remove .travis/images.sh script as molecule now uses ansible for provisioning
• add info about IRC channel to CONTRIBUTING.md
• two test scenarios (default variables and changed ones)

Molecule now uses its own directory, which has following structure:

├── default           # Directory containing test scenario (scenario name is in molecule.yml)
│    ├── create.yml   # Playbook used for environment creation - containers provisioning
│    ├── destroy.yml  # Playbook to cleanup everything after tests
│    ├── molecule.yml # Molecule configuration for this test scenario
│    ├── playbook.yml # Actual tests playbook, role provisioning
│    ├── prepare.yml  # Not used, usually used for environment preconfiguration after creation
│    └── tests
│        └── test_default.py     # testinfra tests
└── alternative
    ├── molecule.yml
    ├── playbook.yml
    └── tests
        └── test_alternative.py

Apart from this it uses .yamllint file for linter configuration.

Multiple test scenarios will significantly extend CI test time. This could be parallelized by running each scenario in different travis pipeline. However this will result in 14 parallel travis pipelines, and travis-ci.org won't allow more than 5 concurent pipelines, so I don't think this will help much.

[lock]

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