Both function are doing exactly the same thing, the only difference
being the command used.

Create a connection() function, and call it from open_connection() and
secured_connection().

Compared to the previous implementation, the connection() function is
more linear and does not rely on boolean return codes to decide what to
do. Instead, it's trying to do things and expect exceptions to be raised
in case of problem (this is why it's using subprocess.run(...,
check=True) for instance)

Parts of the wifi_nmcli_test.py script was implementing a basic retry
mechanism. Remove this to use the retry decorator instead.

In addition, raise exceptions rather than trying to catching them, or
using boolean return codes.

This is to avoid waiting for the actual retry decorator implementation.