provider/openstack: Add support for updating Subnet Allocation Pools

[jtopjian]

This commit adds the ability to update a subnet's allocation pool.

Fixes #10942

[jtopjian]

==> Checking that code complies with gofmt requirements...                                      
go generate $(go list ./... | grep -v /terraform/vendor/)                                       
2017/05/23 23:42:46 Generated command/internal_plugin_list.go                                   
TF_ACC=1 go test ./builtin/providers/openstack -v -run=TestAccNetworkingV2Subnet -timeout 120m  
=== RUN   TestAccNetworkingV2Subnet_importBasic                                                 
--- PASS: TestAccNetworkingV2Subnet_importBasic (45.99s)                                        
=== RUN   TestAccNetworkingV2Subnet_basic                                                       
--- PASS: TestAccNetworkingV2Subnet_basic (54.13s)                                              
=== RUN   TestAccNetworkingV2Subnet_enableDHCP                                                  
--- PASS: TestAccNetworkingV2Subnet_enableDHCP (45.78s)                                         
=== RUN   TestAccNetworkingV2Subnet_disableDHCP                                                 
--- PASS: TestAccNetworkingV2Subnet_disableDHCP (43.41s)                                        
=== RUN   TestAccNetworkingV2Subnet_noGateway                                                   
--- PASS: TestAccNetworkingV2Subnet_noGateway (45.25s)                                          
=== RUN   TestAccNetworkingV2Subnet_impliedGateway                                              
--- PASS: TestAccNetworkingV2Subnet_impliedGateway (46.12s)                                     
=== RUN   TestAccNetworkingV2Subnet_timeout                                                     
--- PASS: TestAccNetworkingV2Subnet_timeout (45.69s)                                            
PASS                                                                                            
ok      github.com/hashicorp/terraform/builtin/providers/openstack      326.409s                

[stack72]

LGTM :) Thanks for this!

[ghost]

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