Designate DNS - "serial" (Designate metadata field) is changed on Zone's description update

[Lilach Avraham]

### Scenario ###

1) create zone example.com. with:
openstack zone create --email <email address hidden> example.com.

+----------------+--------------------------------------+
| Field | Value |
+----------------+--------------------------------------+
| action | CREATE |
| attributes | |
| created_at | 2022-02-09T15:56:41.000000 |
| description | None |
| email | <email address hidden> |
| id | 8c590350-a44b-48ea-b031-3a8e1311ba89 |
| masters | |
| name | example.com. |
| pool_id | 794ccc2c-d751-44fe-b57f-8894c9f5c842 |
| project_id | ef9d9ca443674056b1f8b7f642313b7b |
| serial | 1644422200 |
| status | PENDING |
| transferred_at | None |
| ttl | 3600 |
| type | PRIMARY |
| updated_at | None |
| version | 1 |
+----------------+--------------------------------------+

2) Update zone's description with:
openstack zone set --description 'some description' example.com.

+----------------+--------------------------------------+
| Field | Value |
+----------------+--------------------------------------+
| action | UPDATE |
| attributes | |
| created_at | 2022-02-09T15:56:41.000000 |
| description | some description |
| email | <email address hidden> |
| id | 8c590350-a44b-48ea-b031-3a8e1311ba89 |
| masters | |
| name | example.com. |
| pool_id | 794ccc2c-d751-44fe-b57f-8894c9f5c842 |
| project_id | ef9d9ca443674056b1f8b7f642313b7b |
| serial | 1644422527 |
| status | PENDING |
| transferred_at | None |
| ttl | 3600 |
| type | PRIMARY |
| updated_at | 2022-02-09T16:02:07.000000 |
| version | 3 |
+----------------+--------------------------------------+

### Expected result ###
"Serial" value doesn't changed, "Description" field (unlike TLL or Email) is only known to the Designate and not propagated to the Nameservers.

### Actual result ###
We do see that the serial number has changed on "Description" field update.
from : | serial | 1644422200 |
to : | serial | 1644422527 |