Stylistic improvement of models.ComputeNodeStat

The patch is aimed at improving the readability of models.ComputeNodeStat.

Here, we rename the 'stats' attribute of the class to 'compute_node' (due to
the fact that the corresponding relation actually maps to ComputeNode class,
not to ComputeNodeStat). We also rewrite the relationship() method call to
make it stylistically conforming with the analogous 'service' relationship
in the ComputeNode class: we add the 'foreign_keys' parameter, wrap the
'backref' parameter into a method call and remove the auxiliary 'primary_join'
variable.

Change-Id: I2b1f27b4c1d06f364af01fc82d3490fd3bbf4ac0
Closes-Bug: #1215390
Merge "Stylistic improvement of models.ComputeNodeStat"