fix issues with some malfunctioning docker commands (#2309)

[dg1223]

The following Docker commands now work correctly:

d = Docker()
d = Docker('fedora:latest')
d = Docker(image='fedora:latest')
d = Docker(image='fedora:latest', mode='P1')

In addition, added unit tests for each of these commands.

Please note that this fix does not remove the __init__ function from Docker,py as suggested here. Instead, it sets default values for the image and mode parameters in the __init__ function in Docker.py.

[egede]

@dg1223 Thanks for adding the test cases as well.

[dg1223]

@dg1223 Thanks for adding the test cases as well.

No problem at all!