Implement checking for the master role in rapi

This patch moves the CheckMaster function from ganeti-masterd to ssconf
(most logical place, it cannot go in utils since we would have recursive
imports between ssconf and utils) and changes ganeti-rapi to also call
this function.

This is needed so that starting ganeti-rapi on a non-master node does
the right thing.

Reviewed-by: ultrotter