[feature] Add SIGUSR2 which forces the reload of the config

[nemesifier]

We could listen to SIGUSR2 which would be like SIGUSR1 but with the difference that the following files will be removed first:

• /tmp/openwisp/checksum
• /etc/openwisp/checksum

This will force the reload of the configuration.

It would be super great to also provide some sort of shortcut to send these signals to the agent, easily, with a command that we can remember, what about:

openwisp_config --check-conf
openwisp_config --force-conf

I find myself doing these kind of things all the time manually, I am sure many users do the same or struggle to find out how to do this, so if we add an easy way and document it, it will be useful.