varnish module: add instance_name option

[ilyam8]

Summary

Fixes: #5262

Component Name

/collectors/python.d.plugin/varnish

Additional Information

Details in #5262 (comment)

[ilyam8]

@feld

please check the fix

[feld]

I will test it soon, but this looks correct:

        if self.instance_name:
	            self.command = [varnishstat, '-1', '-n', self.instance_name]
	        else:
	            self.command = [varnishstat, '-1']
	        return True

[ilyam8]

@feld ye, it is an easy fix, and i am sure it will be working, but still better to check

[ilyam8]

@feld any news on this?

[ilyam8]

@cakrit please approve