Adding uri to entity update

I figured this was worth it's own PR because it is added functionality that requires the changes in #52

With an impersonation token from identity with the correct monitoring role. Usage example:

``raxmon-entities-update --api-token **** --tenant 123456 --id enofGJxztQ --uri https://dfw.servers.api.rackspacecloud.com/123456/servers/abcdef-123a-4a80-9bec-ce8c9b329946``