Reuse HTTP client pool for RPC

ganeti-masterd: Add initialization and shutdown of RPC pool. It needs
to be shutdown before forking.

ganeti.cli: Add decorator function to initialize and shutdown RPC pool.

ganeti.rpc: Add functions to initialize and shutdown RPC pool. Throw
exception when used without proper initialization.

gnt-cluster, gnt-node: Use decorator function to initialize and shutdown
RPC pool.

Reviewed-by: iustinp