Tech Debt: Plan uri construction should happen in Plan

[Noorul Islam K M]

See the comments in

https://review.openstack.org/#/c/79997/3/solum/api/controllers/v1/datamodel/assembly.py

Also subsequent discussion on IRC

<asalkeld> if that goes in I can post some more patches
<julienvey> noorul: about plan_uri, there will be a case where a plan is
            stored externally [10:05]
<julienvey> noorul: in this case, we will not construct the plan_uri but just
            return the uri of the external plan
<asalkeld> yip
<asalkeld> good point
<noorul> where are we storing uri of external plan? [10:08]
<asalkeld> we are not yet
<asalkeld> :( [10:09]
<asalkeld> but we should

[Julien Vey (vey-julien)]

I will work on the "external plan" part

[Angus Salkeld (asalkeld)]

also see: https://bugs.launchpad.net/solum/+bug/1291114

[Murali Allada (murali-allada)]

We're working on a new resource called 'app' to be consistent with the CLI changes we've made. This bug is no longer needed.