When Nova is deployed with a mix of x86 and arm systems
(for example), it may be necessary to deploy both 'novnc' and
'serialconsole' proxy services on the same host in order to
service the mixed compute estate.
This patch introduces a list which defines the required proxy
console types.
Change-Id: I93cece8babf35854e5a30938eeb9b25538fb37f6