OS::Quantum::Subnet resource, add enable_dhcp

Add enable_dhcp to the properties schema, since quantumclient
code indicates this is allowed in the body (not only expected
in the response as currently indicated by the docs)

Fixes bug #1194394

Change-Id: I19770b033696ea9a27b6e985a52bf2cfadb16f52
Merge "OS::Quantum::Subnet resource, add enable_dhcp"