Add support for selecting service template

[pdorschner]

Implements configurable service template ref: vshn/signalilo#96

See the notes in the PR from vshn/signalilo#116

[simu]

In the current state, this PR is not mergeable, as it moves a bunch of Signalilo-specific Service details into this library, which is intended to be a generic client for the Icinga2 API.

Additionally, the PR removes the ability to inspect a service's custom variables as returned from the Icinga2 API completely. This is a clear regression which needs to be addressed, see inline comments for options on how to do this.

[pdorschner]

I've implemented new helper functions, which flattens the vars, so now it's possible to post all customvars in one level

[pdorschner]

Hi,
is there any chance that this PR will be merged?

Best Regards

[simu]

It's on my TODO list to review this PR and the corresponding Signalilo PR. I'll try to get it done this or next week.

[simu]

Implementation LGTM now, except for the inline comment regarding the missing Vars contents when using GetService() or ListService().

[pdorschner]

All issues have been fixed