Add command to print variables

[markusressel]

For debugging purposes it would be nice to have a command that will print the variables that can be used in templates, given a path within the openhasp-configs directory (or its root). The output should be formatted in a way that can be easily copied and pasted into a jinja template within the config.

Example:

> openhasp-config-manager vars -c ./openhasp-configs -p touch_down_1/home
key.subkey: "value"
key.othersubkey: 10