Improve apt packaging to better handle directories the agent needs and allow clean uninstall and reinstall

[soenkeliebau]

The improvements that were made to the rpm package in #329 need to be checked in and ported to the apt packaging mechanism if necessary.

Specifically this means:

• The package should be installable without an error when /etc/stackable/stackable-agent/secret exists and contains keys
• The package should remove /opt/stackable/stackable-agent as well as the binary file in there when it is uninstalled
• The package should fully remove /etc/stackable/stackable-agent if only the default config file exists in that directory when it is uninstalled
• The package should correctly declare /etc/stackable/stackable-agent/agent.conf as a config file so that config file update mechanisms handle changes to that file in the proper way