Make Server CONFIG_DRIVE property a boolean

There's no reason for CONFIG_DRIVE to be a string.  It only accepts a
boolean.

Related change in python-novaclient:
https://review.openstack.org/#/c/91169

Change-Id: I2ff588537fbe72fefed0163b69e68c4f484a38e7

heat/engine/resources/server.py

@@ -273,8 +273,8 @@ class Server(stack_user.StackUser):
             _('A UUID for the set of servers being requested.')
         ),
         CONFIG_DRIVE: properties.Schema(
-            properties.Schema.STRING,
-            _('value for config drive either boolean, or volume-id.')
+            properties.Schema.BOOLEAN,
+            _('If True, enable config drive on the server.')
         ),
         DISK_CONFIG: properties.Schema(
             properties.Schema.STRING,