[bug] CA/Cert creation generates 2 SQL write queries instead of 1

[nemesifier]

This code:

django-x509/django_x509/base/models.py

Lines 206 to 217 in c5a5acd

	def save(self, *args, **kwargs):
	generate = False
	if not self.pk and not self.certificate and not self.private_key:
	generate = True
	super().save(*args, **kwargs)
	if generate:
	# automatically determine serial number
	if not self.serial_number:
	self.serial_number = self._generate_serial_number()
	self._generate()
	kwargs['force_insert'] = False
	super().save(*args, **kwargs)

Generates two queries for creating a certificate and a CA.

I figured this out while working on openwisp/openwisp-controller#455.

I think this part could be optimized.