Add a function to obtain the master nodes
Add a function to obtain the master nodes

...and not only the name of the master node. The result is
reported as a list of nodes, which is not only convenient
to deal with inconsistent cluster data, it also can be
used for RPCs directly in this form.

Signed-off-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>
Reviewed-by: Petr Pudlak <9d19625f8ee825a2c398462ed335cbcf98f79c32@google.com>

ganeti/ganeti,andir/ganeti,yiannist/ganeti,bitemyapp/ganeti,mbakke/ganeti,mbakke/ganeti,leshchevds/ganeti,yiannist/ganeti,ganeti-github-testing/ganeti-test-1,apyrgio/ganeti,leshchevds/ganeti,apyrgio/ganeti,grnet/snf-ganeti,mbakke/ganeti,dimara/ganeti,ganeti-github-testing/ganeti-test-1,bitemyapp/ganeti,grnet/snf-ganeti,andir/ganeti,onponomarev/ganeti,andir/ganeti,ganeti/ganeti,onponomarev/ganeti,dimara/ganeti,yiannist/ganeti,leshchevds/ganeti,ganeti/ganeti