Adds high availability management-plane support

Support for standby management-node(s), where one is automatically promoted to master when the current master fails or terminates.

See javadoc in HighAvailabilityManagerImpl for an overview of the design (see https://github.com/aledsage/brooklyn/blob/feature/managementNode-warmStandby/core/src/main/java/brooklyn/management/ha/HighAvailabilityManagerImpl.java)

Things missing from this PR (to be done subsequently) include:
* cntrl-c: no shutdown hook registered that calls mangementContext.terminate. This means we don't gracefully notify other nodes we're stopping, so they take the default 30 seconds timeout time before the standby is promoted. It should be instantaneous.
* nothing in the web-console (but there is basic stuff in the REST api)
   * if in "standby", we should not don't open the dialog asking which app to deploy
   * show high-availability status in a suitable tab somewhere in the UI

And much longer term, we should be using the brooklyn entity representing the master+standby nodes, with brooklyn policies for who/when to promote to master.