Move SetKey to WritableSimpleStore and use it

Before we used to be able to update SimpleStore by just calling SetKey, this
feature is now moved to an external class, which inherits from it. In this
patch the new WritableSimpleStore class is also put to use, in the LUs that
need it. Rather than making each LU instantiate it, we have a new LogicalUnit
flag REQ_WSSTORE which defaults to False, but when declared to be True asks the
LogicalUnit to be initialized with a writeable version of the SimpleStore.
LUMasterFailover and LURenameCluster are then changed to use it.

InitCluster is also changed to instantiate a WritableSimpleStore, rather
than a normal one.

Reviewed-by: imsnah