Allow ensure to be undef

[robinbowes]

We don't want puppet to always alter the state of the service - we rely on external orchestration to bring up services in a specific order in many cases.

This change allows ensure to be set to undef.

[juniorsysadmin]

@robinbowes With managing a service you generally want to manage all aspects of it, or not at all. The preferred way to do this is generally with a boolean service_manage parameter. eg. https://github.com/puppetlabs/puppetlabs-postgresql/blob/master/manifests/server/service.pp#L15

[robinbowes]

"generally want to" - perhaps. This change adds flexibility, without changing default behaviour.

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