Add an option for node setup to remove the default global zones

[RincewindsHat]

Is your feature request related to a problem? Please describe.

When adding Icinga2 agents to an Icinga setup, the local Icinga2 on the agent has to be
configured once.
If there is no proper configuration management in place, a relatively simple script
using the icinga2 node setup command will do most of the time, except for on thing:
It is not possible to remove the default global zones (director-global and global-templates)
this way.

This has to be done in another separate step, which seems unnecessary since it would be
easier and more consistent if one could do that with the one command which configures
all the other zone and endpoint stuff.

Describe the solution you'd like

A commandline flag like --remove-default-global-zones to NOT generate
the default zones in the zones.conf file

Additional context

For a best practice setup I remove the default global zones from agents and replace them with
specialised on, which only contain CheckCommand definitions necessary on the agent.
This reduces the amount of config which needs to be deployed and also the reloads.