Some small improvements to the hooks environment

For the configuration update hook, it's useful to have a consistent name
for the target of the operation. As such, the LU code is modified to
include an GANETI_OP_TARGET that points either to the cluster (name),
node name or instance name depending on the opcode.

Also, the NoHooksLU is modified such that its build env method returns
an empty (but conformant) result. This should improve things in case by
mistake this class' BuildHooksEnv is called.

Reviewed-by: imsnah