objects.Cluster: add method to get hv defaults

Currently the FillHV method is the one that does the cluster hvparams +
os hvparams merger. However, in some cases we need to do just this,
without adding the instance hvparams on top.

This patch adds a function to compute just this (hv + os hvp
combination) default dict, and modifies FillHV to use it to build the
final dict.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>