Make PortalIP alloc HA

* Add an allocator which saves state in etcd
* Perform PortalIP allocation check on startup and periodically afterwards

Also expose methods in master for downstream components to handle IP allocation
/ master registration themselves.

docs/services.md

@@ -316,6 +316,15 @@ In order to allow users to choose a port number for their `Services`, we must
 ensure that no two `Services` can collide.  We do that by allocating each
 `Service` its own IP address.
 
+To ensure each service receives a unique IP, an internal allocator atomically
+updates a global allocation map in etcd prior to each service. The map object
+must exist in the registry for services to get IPs, otherwise creations will
+fail with a message indicating an IP could not be allocated. A background
+controller is responsible for creating that map (to migrate from older versions
+of Kubernetes that used in memory locking) as well as checking for invalid
+assignments due to administrator intervention and cleaning up any any IPs
+that were allocated but which no service currently uses.
+
 ### IPs and Portals
 
 Unlike `Pod` IP addresses, which actually route to a fixed destination,