cluster verify: add hvparams verification

Currently, the validity of the hypervisor parameters is only checked
at init/modification time, and not in the cluster verify. This is bad,
as it can lead to inconsistent state that is only detected when the
next modification (which can be unrelated) is made, leading to
unexpected error messages.

This patch adds both syntax verification (in masterd) and validity
verification on remote nodes. The downside of the patch is that on
clusters with many instances which have custom parameters, it will be
slow. A possible improvement would be to detect duplicate, identical
set of parameters, and collapse these into a single verification, but
that is left as a TODO (in case it becomes problematic).

An additional change is in utils.ForceDict, where we said 'key',
whereas this function is always used with parameter dicts, so I
changed it to "Unknown parameter".

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