Make Service objects compute_node attribute a list

In order to make resource tracker use service object, service object
must first be changed to support multi-node. Currently it stores the
first compute node from the list returned by the db.

Related to blueprint bp/kilo-objects

Change-Id: Ie66864ed288cce07535b719da7023539ba2fcb38
Make resource tracker use service object

Change-Id: Ibcab3cb6bdcdb0a20eb6b8863c684f943d3e8af7
service to objects

Change-Id: I87b077699c2ecbc25a83e5844e8c69fe147cec01
Remove and deprecate conductor service calls

Change-Id: Ic099bb239e38242206cb0228823d83cd56d5d3f3