installation: CLI doesn't work when installed in editable mode

```console
$ docker-services-cli --version
docker-services-cli, version 0.2.1
```

## Describe the bug
`docker-services-cli` works nicely when installed in production mode (`pip install docker-services-cli` or `pip install .` when inside the package sources dir). However, an exception is thrown when using the tool installed in editable mode (`pip install -e .` when inside the package sources dir).
## Steps to Reproduce
```console
$ pip install -e .
Obtaining file:///Users/marco/code/inveniosoftware/docker-services-cli
Collecting click<8.0,>=7.0
Using cached click-7.1.2-py2.py3-none-any.whl (82 kB)
Installing collected packages: click, Docker-Services-CLI
Running setup.py develop for Docker-Services-CLI
Successfully installed Docker-Services-CLI click-7.1.2
WARNING: You are using pip version 20.2.4; however, version 20.3.1 is available.
You should consider upgrading via the '/Users/marco/.virtualenvs/docker-services-cli/bin/python -m pip install --upgrade pip' command.
$ docker-services-cli up postgresql12
Usage: docker-services-cli [OPTIONS] COMMAND [ARGS]...
Try 'docker-services-cli --help' for help.

Error: Invalid value for '--filepath' / '-f': Path '/Users/marco/.virtualenvs/docker-services-cli/lib/python3.8/site-packages/docker_services_cli/docker-services.yml' does not exist.
$ pip install .
Processing /Users/marco/code/inveniosoftware/docker-services-cli
Requirement already satisfied: click<8.0,>=7.0 in /Users/marco/.virtualenvs/docker-services-cli/lib/python3.8/site-packages (from Docker-Services-CLI==0.2.1) (7.1.2)
Building wheels for collected packages: Docker-Services-CLI
Building wheel for Docker-Services-CLI (setup.py) ... done
Created wheel for Docker-Services-CLI: filename=Docker_Services_CLI-0.2.1-py2.py3-none-any.whl size=12266 sha256=a64fa2a771856e6dfe1510edf795649d14ea12525fd4dcc43ce9bfa7f4b5db63
Stored in directory: /Users/marco/Library/Caches/pip/wheels/2b/3c/85/b3a3a4904a0243297086e24fa3e62ed49e9d00f92f1ba9a5e0
Successfully built Docker-Services-CLI
Installing collected packages: Docker-Services-CLI
Attempting uninstall: Docker-Services-CLI
Found existing installation: Docker-Services-CLI 0.2.1
Uninstalling Docker-Services-CLI-0.2.1:
Successfully uninstalled Docker-Services-CLI-0.2.1
Successfully installed Docker-Services-CLI-0.2.1
WARNING: You are using pip version 20.2.4; however, version 20.3.1 is available.
You should consider upgrading via the '/Users/marco/.virtualenvs/docker-services-cli/bin/python -m pip install --upgrade pip' command.
$ docker-services-cli up postgresql12
Environment setup
Creating network "docker_services_cli_default" with the default driver
Creating docker_services_cli_postgresql_1 ... done
postgresql up and running!
Services up!
```

## Expected behavior
```console
$ pip install -e .
Obtaining file:///Users/marco/code/inveniosoftware/docker-services-cli
Collecting click<8.0,>=7.0
Using cached click-7.1.2-py2.py3-none-any.whl (82 kB)
Installing collected packages: click, Docker-Services-CLI
Running setup.py develop for Docker-Services-CLI
Successfully installed Docker-Services-CLI click-7.1.2
WARNING: You are using pip version 20.2.4; however, version 20.3.1 is available.
You should consider upgrading via the '/Users/marco/.virtualenvs/docker-services-cli/bin/python -m pip install --upgrade pip' command.
$ docker-services-cli up postgresql12
Environment setup
Creating network "docker_services_cli_default" with the default driver
Creating docker_services_cli_postgresql_1 ... done
postgresql up and running!
Services up!
```

## Additional context

Originally reported by @mvidalgarcia in https://github.com/inveniosoftware/docker-services-cli/pull/16#pullrequestreview-544828561.