Allow user defined system streams

See discussion in https://mail-archives.apache.org/mod_mbox/incubator-storm-user/201407.mbox/%3C1405958743.31781.144006701.0C306616%40webmail.messagingengine.com%3E

We currently get an exception when defining a stream starting with "__". There are cases where we might want to be able to do that, e.g. treat a stream as a system one so that its stats can be hidden in the UI just like any other system stream.

As suggested by Harsha in the mail thread above

we can probably add a exception in nimbus if a stream starts with "__" and its in storm.user.system.streams.