NodeInfo remove main identity (#1284)

* Remove node's main identitiy from NodeInfo.

Preparation for getting rid of services + supporting multiple identities
on the node.
NodeInfo keeps multiple identities as a list. For now the first one is treated as a special one.
Introduced function chooseIdentity in CoreTestUtils as a preparation for proper handling of multiple identities in the future.
Remove legalIdentityKey from ServiceHub, add extension function - chooseIdentity on ServiceHub.
Add `me` field on FlowStateMachineImplemetation, flows should know what the calling identity is.
Remove SERVICES_PREFIX in artemis messaging layer.

* Address minor comments.

* Fixes after rebase.

Remove chooseIdentity from ServiceHub

* Rename me to ourIdentity on FlowLogic

* Fixes after rebase

* Address Ross comments, fixes

* Fix after rebase

* Fix services certificate paths

Apply Patrick's patch.

webserver/src/main/kotlin/net/corda/webserver/internal/APIServerImpl.kt

@@ -18,5 +18,5 @@ class APIServerImpl(val rpcOps: CordaRPCOps) : APIServer {
         return Response.ok("started").build()
     }
 
-    override fun info() = rpcOps.nodeIdentity()
+    override fun info() = rpcOps.nodeInfo()
 }