Allow to add groups to templates

[tobiasvdk]

Expected Behavior

I'm able to add groups to an e.g. host template which already inherits a group.

Current Behavior

Inherited groups are overridden when a group is configure in a template.

Possible Solution

Add a checkbox to "Add groups" which results in rendering

groups += ["hg2"]

Steps to Reproduce (for bugs)

1. Create host template "template1" and add a host group "hg1"
2. Create a new host template "template2" and add a host group "hg2"
3. The host template "template2" has only the host group "hg2"

Your Environment

• Director version (System - About): 1.5.1
• Icinga Web 2 version and modules (System - About): 1.6.2
• Icinga 2 version (icinga2 --version): 2.10.2
• Operating System and version: Debian Stretch
• Webserver, PHP versions:

[tobiasvdk]

This feature is probably already meant in #824.

[lesinigo]

And probably also in #344