Store per-master-component command line flags in etcd, by creating a "MasterFlags" API directory (issue #1627).

command line flags (issue #1627).
This is actually more of a placeholder than anything else. I'd like to replace this with direct read from etcd (only for the apiserver reading flags, not the other master components), but couldn't figure out how to do it. I'm hoping one of the more seasoned folks can point me in the right direction.