executor: add support for custom ansible_port

This change adds 'ssh_port' to the Node class so that zuul-executor can
use a custom ssh server when it is set by nodepool provider.

Change-Id: Icdac6cd41dded0e46fba5d14c31f40810f73b74a
diff --git a/zuul/model.py b/zuul/model.py
index 610b9b6..1496268 100644
--- a/zuul/model.py
+++ b/zuul/model.py
@@ -367,6 +367,7 @@
         self.public_ipv4 = None
         self.private_ipv4 = None
         self.public_ipv6 = None
+        self.ssh_port = 22
         self._keys = []
         self.az = None
         self.provider = None