(MAINT) Make Port Check Configurable

This change makes the port and port timeout configurable via the YAML
config file. Before these were hard coded to 22 and 5, respectively.

Tests have been added to verify the default is chosen when not
configured and the configuration is picked up when supplied.

vmpooler.yaml.example

@@ -161,6 +161,16 @@
 #
 #   - domain
 #     If set, returns a top-level 'domain' JSON key in POST requests
+#
+#   - port
+#     The port number used to verify a host is ready and available.
+#     (optional; default: 22)
+#
+#   - port_timeout
+#     How long (in seconds) to wait for connection to port before canceling.
+#     This is not used in marking a VM dead or stale.
+#     (optional; default: 5)
+#
 
 # Example:
 
@@ -176,6 +186,8 @@
     - 'created_by'
     - 'project'
   domain: 'company.com'
+  port: 22
+  port_timeout: 5
 
 # :pools:
 #