qgen: model INSERT query

The random query generator builds and inspects composition of a query by building a tree of Python objects. The sql writer uses this model to then write the query.

INSERT gets its own Jira because it's the first.

The rough task list/design is something like:

1. Create an abstract Query() class so that all queries areinstancesof() Query.
2. Make current Query() a SelectQuery()
3. InsertClause class with required kudu table attribute
4. ValuesClause has a required column list and a values list
5. InsertQuery will have a required InsertClause and a required SelectQuery xor ValuesClause

It may be enough for this task to stub them out and fill in functionality when actually generating or writing queries.