Fix redistribute config and offline nodes

We need to manually filter out offline nodes before using
rpc.call_upload_file and rpc.call_write_ssconf_files, since these method
are static (they work without a ConfigWriter instance) and thus do not
know which nodes are offline and which are not).

Note that we add a new ConfigWriter._UnlockedGetOnlineNodeList() method
rather than hardcoding the filtering of online nodes in _WriteConfig.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>