Add cluster.SimpleFill*() functions

Currently, the existing cluster.Fill* functions take as argument an
instance. This means that in any case where we don't have an actual
instance object, we have to resort to calling the low-level
objects.FillDict function.

This is bad for two reasons:
- we have to know of, and we hardcode, the cluster object internals
  (e.g. that the nicparams are stored in a dict indexed by group)
- which can result in subtle bugs, if the underlying storage mechanisms
  change

This patch adds a lower-level implementation SimpleFillHV for FillHV and
SimpleFillBE for FillBE, and adds a completely new SimpleFillNIC (all
use cases until now hardcoded cluster.nicparams[constant.PP_DEFAULT]
directly); it then uses these new functions in cmdlib.py.

A side effect is that _CheckNicsBridgesExist loses the 'profile'
parameter, which was unused. If it's needed, we should add it later via
a proper profile parameter to SimpleFillNIC.

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