Create base class for handling cloud providers

Defined in providers module in base.py, this class provides
general interface for handling multiple cloud providers.

Signed-off-by: Shashank Sahni <shredder12@gmail.com>
Added ec2 provider class

Refactored previous ec2 implementation to create a new EC2 class
derieved from BaseProvider.

Signed-off-by: Shashank Sahni <shredder12@gmail.com>