We have a bunch of code duplication and some shady business about naming the executors in nova.utils. We should clean this up

Acceptance criteria:

• There is a single interface to create executors
• There is a single call to shut down all executors created by the process
• Executor definition is moved from nova.utils to its own module
• Name should be part of the Executor interface
• Globalness of the executors more encapsulated
• Code duplication is removed