Cluster verify: verify hypervisor parameters only once

The list of all hypervisor parameters has to be computed in
LUClusterVerifyGroup, since it needs to be passed to nodes as
NV_HVPARAMS. However, it is better only to verify said parameters once,
out of LUClusterVerifyConfig.

For this, we refactor the code that constructs the list of parameters to a
module-level _GetAllHypervisorParameters() function that both LUs can use.

Signed-off-by: Adeodato Simo <dato@google.com>
Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>