Allow for multi-instance 'failover' configuration of hstRestProxyService

When configuring multiple nodes for a clustered setup, the rest.uri of hstRestProxyService is problematic to configure. It allows for only one url with host port (like: http://127.0.0.1:8080/site/_cmsrest) When running two nodes with different IP's it enforces the use of some sort of proxy to do failover. When setting up so the rest stays within the dmz it requires extra configuration.
Even for the infra-setup of the test server this is fixed to one instance so in case of failure the channel manager doesn't work.

Possible solutions to make this better:

• Allow for determining the default host/port to be the same as the current CMS instance. This makes sure the same node is used.
• Allow for multiple url's, for example 'url1,url2' so in case of failure the next url is picked.
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