Remove instances property of agentil_template

Template customizations like the JOB177 (instance availability) was
managable with the `agentil_template` type. This has a major drawback:
When defining the agentil_template you already have to know all your
possible customizations.

Example: When you run a SAP system with three application server each
running two instances you can monitor these with a single template. In
order to avoid duplicate resources you are know defining your
`agentil_template` only for your message server. If you know want to
customize job 177 (instance availablity) your message server must have
knowledge of all other instances running on the other two servers.

So remove the instances property. Customizations should be manageable
with a different puppet type.
Add an interface to agentil_template to manage customizations

A system template can define customizations for specific jobs (e.g. the
correct names of your SAP instances for job177 [instance availability])

To allow these template customizations to be managed through puppet,
a template now tracks the current customizations and allows to destroy
and create new customizations.

This interface can be used in other custom puppet types that want to
manage different customizations (e.g. a future `agentil_instance` could
be managing customizations for job177 [instance availability])
Add a new type agentil_instance

The `agentil_instance` type can be used to manage the customization of
job 177 (instance availablility).
Merge branch 'agentil_instance'

* agentil_instance:
  Add a new type agentil_instance
  Add an interface to agentil_template to manage customizations
  Remove instances property of agentil_template