Add default-quota to subnet pool commands

Add --default-quota option to subnet pool create and set commands.

Setting default-quota back to None may break the current Neutron
behavior, therefore support for Unset command is not provided in
this patch.

Neutron API:
a0e0e8b668/neutron/api/v2/attributes.py (L239)

Closes-Bug: #1667294
Change-Id: Ia4e7c23a49e91a090133c729353cdb8e62bc5674

doc/source/command-objects/subnet-pool.rst

@@ -24,6 +24,7 @@ Create subnet pool
         [--address-scope <address-scope>]
         [--default | --no-default]
         [--share | --no-share]
+        [--default-quota <num-ip-addresses>]
         --pool-prefix <pool-prefix> [...]
         <name>
 
@@ -73,7 +74,12 @@ Create subnet pool
 
     Set this subnet pool as not shared
 
-.. describe:: --pool-prefix <pool-prefix>
+.. option:: --default-quota <num-ip-addresses>
+
+    Set default quota for subnet pool as the number of
+    IP addresses allowed in a subnet
+
+.. option:: --pool-prefix <pool-prefix>
 
     Set subnet pool prefixes (in CIDR notation)
     (repeat option to set multiple prefixes)
@@ -169,6 +175,7 @@ Set subnet pool properties
         [--address-scope <address-scope> | --no-address-scope]
         [--default | --no-default]
         [--description <description>]
+        [--default-quota <num-ip-addresses>]
         <subnet-pool>
 
 .. option:: --name <name>
@@ -213,6 +220,11 @@ Set subnet pool properties
 
     Set subnet pool description
 
+.. option:: --default-quota <num-ip-addresses>
+
+    Set default quota for subnet pool as the number of
+    IP addresses allowed in a subnet
+
 .. _subnet_pool_set-subnet-pool:
 .. describe:: <subnet-pool>