Changing a Ganeti node's group

# gnt-group list
Group    Nodes Instances AllocPolicy NDParams
tier1     5        78 last_resort (empty)
tier2     3       118 preferred   (empty)
mvb       4        74 last_resort (empty)
# gnt-group assign-nodes cc-tier2 sever
# gnt-group list
Group    Nodes Instances AllocPolicy NDParams
tier1     5        78 last_resort (empty)
tier2     4       124 preferred   (empty)
mvb       3        68 last_resort (empty)