Docker inventory script tls connection support

This PR adds TLS connection support for the docker inventory script.

I have come across the problem of the docker-inventory script not working any more with boot2docker 1.3+.
The issue is that boot2docker creates a server that wants to use a full TLS mutually authenticated connection (e.g. server cert+client cert).
The current docker inventory script does not support TLS connection settings setup.

Notable changes:

- Now you can use the script to inventorize one docker instance by having the configuration variables stored in shell environment variables.

    boot2docker example:

    (By default boot2docker will use tls as of version 1.3.0)

        boot2docker init
        eval "$(boot2docker shellinit)"
        export DOCKER_ASSERT_HOSTNAME="boot2docker"
        ./docker.py --list

- Now the connection settings stored in shell environment variables are imported using docker-py's docker.utils.kwargs_from_env() function. That properly sets up the kwargs (including the docker.tls.TLSConfig) needed by the Connection constructor.

- The configuration data structures -and so the YAML confguration file- were changed, so that the server connection configuration sits in its own dict.

    Now the script supports multiple connection methods (plain, simple unauthenticated https://, mutually authenticated tls). This means that the docker Client connection __init__ now can be called with may different signatures. Cherrypicking the options can lead to problems: either leaving something out, or calling with an incompatible constellation of options. It's cleaner and more maintainable to keep this settings in a seperate dict, that can be used as kwargs for the Connection constructor.

    Now the variable names in the 'server' configuration stanza correspond to the arguments of the constructor of the docker.Client class, the exception being the tls object. The TLSConfig class is instantiated with the kwargs in the tls_config object.

    For reference, the signature of the docker.tls.TLSConfig __init__:

        def __init__(self, client_cert=None, ca_cert=None, verify=None,
                     ssl_version=None, assert_hostname=None):

    The signature of the docker.Client __init__:

        def __init__(self, base_url=None, version=DEFAULT_DOCKER_API_VERSION,
                     timeout=DEFAULT_TIMEOUT_SECONDS, tls=False)

    Configuration example:

        ---
        - defaults:
            server:
              base_url: https://192.168.59.103:2376
              tls_config:
                client_cert:
                  - /home/me/.boot2docker/certs/boot2docker-vm-docker-dev/cert.pem
                  - /home/me/.boot2docker/certs/boot2docker-vm-docker-dev/key.pem
                ca_cert: /home/me/.boot2docker/certs/boot2docker-vm-docker-dev/ca.pem
                verify: true
                assert_hostname: false
                ssl_version: TLSv1
              version: 1.9
              timeout: 60


- The old-style settings are imported, for the script needs to be backward-compatible. For imported settings, TLS is disabled (as the old configuration had no support for tls settings.) If you have an old-style configuration file and you want tls connections, you will have to convert your settings. If you don't want tls, you don't need to do anything.