Allow setting of first ip for network reservations

[treywelsh]

This is a PR that apply some small changes to #185 authored by @danhigham

[treywelsh]

This PR introduce the reservation_ar_id attribute.
It's easy to retrieve with an already existing AR but we may want to be able to create a reservation at the same time that we create the AR in the parent network.
In addition, it's not possible to rely on address range order to guess the ID (ARs are stored in a TypeSet. We could store them in a TypeList instead, but keep AR ordering through future AR modification is not so funny).

I don't see any easy and clean way to work with reservation_ar_id other than externalize the address range to it's own resource (but it's not the only argument to do this anyway): #279

I'll remove this issue from 0.5.0 milestone, it will require a bit more of work

[treywelsh]

depends on #279 should be scheduled for 1.0 release

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[treywelsh]

rebased on top of #306 until it's merged to make this PR move forward