Add support for using environment variables to pass parameters

[zhan9san]

In debug mode, the command is artifactory-cleanup, but in destroy mode, the command would be artifactory-cleanup --destroy

With environment variable supported, we can use the same command with different environment variable.

The same behavior happens if we want to use different configuration files in different Artifactory servers.

So I propose to add environment variables support, e.g.

• ARTIFACTORY_CLEANUP_CONFIG_FILE
• ARTIFACTORY_CLEANUP_DESTROY
• etc.

https://plumbum.readthedocs.io/en/latest/cli.html#environment-variables