instancehome as base is insufficiant to tag instance_name

[jensens]

This works with one buildout creating several instances and run them on one machine.

It does not work:

• same buildout on several severs
• with docker images

code see

collective.sentry/collective/sentry/error_handler.py

Lines 209 to 212 in 6d8e79d

	configuration = getConfiguration()
	tags = {}
	instancehome = configuration.instancehome
	tags["instance_name"] = instancehome.rsplit(os.path.sep, 1)[-1]

This probably needs some conceptual work to get this right.
I consider this as a bug with low importance.