Add LUSetClusterParams to cmdlib

Add LUSetClusterParams, which is the LU to modify cluster options.
This includes checks:
- not to disable lvm storage when it's already disabled
- not to enable lvm storage when it is already enabled
- not to disable lvm when lvm-based instances are present
- that the specified volume group is valid on all cluster-nodes
  when lvm-storage is going to be enabled

Reviewed-by: iustinp