[change] Use SIGUSR1 signal to reload config during push updates

[nemesifier]

Now that we can trigger openwisp-config to download and apply the config with a SIGUSR1 signal, we can use this new and better way during push updates.

This is what we use nowadays:

openwisp-controller/openwisp_controller/connection/connectors/openwrt/ssh.py

Lines 9 to 16 in 0afef2f

	def update_config(self):
	_, exit_code = self.exec_command(
	'test -f /tmp/openwisp/applying_conf', exit_codes=[0, 1]
	)
	if exit_code == 1:
	self.exec_command('/etc/init.d/openwisp_config restart')
	else:
	logger.info('Configuration already being applied')

We would have to change the code to check if the openwisp-config version is higher or equal to 0.6.0a, in that case we can use the new method (we have to find the pid of openwisp-config and then call kill -SIGUSR1 <pid>.