Introduces eventlet executor for Glance Tasks

A sample import script to successfully import image for filesystem
based backend store is introduced as well.

Also, the following changes are introduced:-
1. An interface for implementing any other type of executor.
2. Provides namespace for keeping Tasks scripts.
3. A config for choosing deployer specific executor.
4. An interface for writing Tasks scripts.
5. A module for common script related methods.
6. Logic for limiting number of simultaneous tasks execution on the
Glance API server.

partially implements bp async-glance-workers

Change-Id: I382472fffd0fdad43573e72b2e78a9a6ed1e1f1a