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
Merge "Make Server CONFIG_DRIVE property a boolean"