Fix compute_node_get_all() for Nova Baremetal

Change Ie5ef00c974b810336787e88c78c93c15ca2890d3 introduced
a regression leading to KeyError when a new baremetal node
is scheduled. This is due to the fact, that the mentioned
change assumes, that ComputeNode <--> Service is a 1-1
relationship, which is not true for Nova Baremetal driver.

This patch fixes the tables join in compute_node_get_all()
DB API method to work with 1-M relationship between ComputeNode
and Service models.

Closes-Bug: #1221620

Change-Id: I7c218d06f63cc2bf7d0e358f2f76366601179b0c
Merge "Fix compute_node_get_all() for Nova Baremetal"