tox: add `cover` environment

This environment runs a test coverage report intended to readable on the
command line.
Cleanup imports

This organizes the imports by sorting and grouping them, and also adds
handling for import ConfigParser in python 2 and 3.
Add models classes for Channel and Manifest

These classes help to generate manifest files and the contained channel
definitions.
Add tests for Channel and Manifest classes
tox: include doctests in test runs

Tox/py.test will now look for doctests in docstrings.
Add util func for processing zvm-image args

The user can specify multiple --zvm-image arguments on the command line;
each image can include optional mount point and access (ro or rw)
parameters, which needs to be written to the fstab in the nvram file.

This function processes the command line arguments into something which
can easily be written into the nvram file.
Add __repr__ for Channel
Add create_manifest util function

This is a helper function to create a `Manifest` object for the zvsh
`run_zerovm` workflow.
zvsh_test: Add a test for Channel.__repr__
zvsh_test: Add a test for `create_manifest` util