enhancement: Skip more tasks when skipping install

[tvenieris]

When not installing, /usr/local/bin/blackbox_exporter is not guaranteed to be present. Which means that applying cap_net_raw capability to it may fail and should be skipped too. This means that installing the libcap2-bin package may be unnecessary, if not contradicting the principle of the 'skip_install' setting. And the same goes for the installation of the python3-apt package in the preflight script.

All of the above are irrelevant when using this role to setup the configuration of a container which comes with everything pre-installed.