Add prefixlen to subnet api-ref for create actions

Prefixlen is used in subnet creation when allocating a subnet
from a subnet pool.

Change-Id: I251944ca15434ed9b17deb829cca985cd94a0bd1
Closes-bug: #1754062

api-ref/source/v2/parameters.yaml

@@ -4665,6 +4665,14 @@ subnet-obj:
   in: body
   required: true
   type: string
+subnet-prefixlen-request:
+  description: |
+    The prefix length to use for subnet allocation from a subnet pool.
+    If not specified, the ``default_prefixlen`` value of the subnet pool
+    will be used.
+  in: body
+  required: false
+  type: integer
 subnet-segment_id:
   description: |
     The ID of a network segment the subnet is associated with.

api-ref/source/v2/subnets.inc

@@ -173,6 +173,7 @@ Request
    - ip_version: subnet-ip_version
    - gateway_ip: subnet-gateway_ip-request
    - cidr: cidr
+   - prefixlen: subnet-prefixlen-request
    - description: description-request
    - ipv6_address_mode: subnet-ipv6_address_mode-request
    - ipv6_ra_mode: subnet-ipv6_ra_mode-request
@@ -252,6 +253,7 @@ Request
    - ip_version: subnet-ip_version
    - gateway_ip: subnet-gateway_ip-request
    - cidr: cidr
+   - prefixlen: subnet-prefixlen-request
    - description: description-request
    - ipv6_address_mode: subnet-ipv6_address_mode-request
    - ipv6_ra_mode: subnet-ipv6_ra_mode-request