Dcos-shakedown

Fixes:

- improved SSH session-handling

Features:

- support for DC/OS 1.11 (`dcos_1_11` PyTest marker)

Fixes:

- don't flake if a connection error occurs during an install

Features:

- new function to retrieve available versions of packages
- `get_package_versions()`
- bumped `dcoscli` version to `0.5.7`

Fixes:

- handle `skip` state when printing output

Features:

- new leader-based run functions
- `run_command_on_leader()`
- `run_command_on_marathon_leader()`
- log the resulting package version when installing from `latest`

Features:

- added API functions to query for Docker version
- `docker_version()`
- `docker_client_version()`
- `docker_server_version()`
- added classifiers to the setup to specify the programming langauge to
be used with pipenv

Fixes:

- removed staticly-set assertions in `test_get_reserved_resources()`

Features:

- all CLI options now have the option to be specified as environmental
variables, eg. `SHAKEDOWN_DCOS_URL`, `SHAKDOWN_USER`, etc.

Fixes:

- CLI subcommand (non-app) packages now treated the same as app/service
packages
- `install_package()` and `uninstall_package()` won't throw exceptions
- `package_installed()` returns `True` for non-app packages