Allow plugin implementation for CREATE and DROP DATABASE

[deepthi]

Feature Description

We should add support for plugin implementations of CREATE DATABASE and DROP DATABASE.

• The default plugin will return an error (NOT_ALLOWED or something like that).
• For CREATE, if database (keyspace) already exists we return an error, otherwise we call the plugin.
• For DROP, if database (keyspace) does NOT exist, we return an error, otherwise we call the plugin.

Use Case(s)

Frameworks like Rails create temporary databases for running test suites.

[systay]

The default plugin should return a failure, not be a no-op, right?

[deepthi]

The default plugin should return a failure, not be a no-op, right?

Good point. I will edit the description.