Drop usage of RpcProxy from DhcpPluginApi

Drop the usage of the RpcProxy compatibility class from the
DhcpPluginApi RPC client class.  The implementation has been updated
to use the appropariate APIs from oslo.messaging directly.

Part of blueprint drop-rpc-compat.

Change-Id: I81bfd761707c4c587b12877668c4399efe3d652e
Fix client side versions in dhcp rpc API

The dhcp rpc API has two version (1.0 and 1.1).  The proper way to use
versioning for this is to only specify '1.1' from the client side when
you require that the remote side implements at least version '1.1' for
the method to work.  Otherwise, '1.0' should still be specified.  The
previous code specified '1.1' always.

Related to blueprint drop-rpc-compat.

Change-Id: I9468f8f67d80c5d064137f917fc04f9335a3ed55