Unable to change master_count bay attribute

The master_count bay attribute can not be changed on a running bay.

Steps to reproduce:

1) Create a baymodel:

stack@demo:~$ magnum baymodel-create --name k8sbaymodel \
                       --image-id fedora-atomic-latest \
                       --keypair-id testkey \
                       --external-network-id public \
                       --dns-nameserver 8.8.8.8 \
                       --flavor-id m1.small \
                       --docker-volume-size 5 \
                       --network-driver flannel \
                       --coe kubernetes

2) Create a k8s bay from that baymodel:

stack@demo:~$ magnum bay-create --name k8sbay --baymodel k8sbaymodel --node-count 1

3) Attempt to change the master_count bay attribute:

stack@demo:~$ magnum bay-update k8sbay replace master_count=2
ERROR: cannot change bay property(ies) master_count (HTTP 400) (Request-ID: req-6722c6b8-6731-46b2-bf0b-5b47c82a50c4)

Expected Result:

The bay's heat stack should be updated to a multi-master configuration.

Actual Result:

The API returns a 400 Bad Request response.