Consider making ovsdb.Operation an interface

[dave-tucker]

It's used a lot as a return type (e.g client.Create return []ovsdb.Operation and as a parameter in client.Transact

Since there are a number of optional fields, constructing operations yourself can be error prone (and annoying when having to create temporary variables to assign to a *string as literals can't be indirected). To avoid leaking this datastructure outside the OVSDB package, we should consider providing an intermediate type in the client package - like we've done for Condition or Mutation for example:

package client

type Operation interface {
    json.Marshaler
    json.Unmarshaler
}

We'd also want functions to create new Operations, e.g NewInsertOperation, NewDeleteOperation, NewCommentOperation