Remove secret_key config, not needed with the cookie session

Docker-DCO-1.1-Signed-off-by: Sam Alba <sam.alba@gmail.com> (github: samalba)
Remove session creation and unused session checks

Docker-DCO-1.1-Signed-off-by: Sam Alba <sam.alba@gmail.com> (github: samalba)
Adapt unit tests to reflect recent changes on Payload-Checksum

Docker-DCO-1.1-Signed-off-by: Sam Alba <sam.alba@gmail.com> (github: samalba)
Remove cookie session code (closes #339 #330)

Docker-DCO-1.1-Signed-off-by: Sam Alba <sam.alba@gmail.com> (github: samalba)
Cleanup some unused deprecated to secret_key

Docker-DCO-1.1-Signed-off-by: Sam Alba <sam.alba@gmail.com> (github: samalba)
Cleanup old references to secret_key

Docker-DCO-1.1-Signed-off-by: Sam Alba <sam.alba@gmail.com> (github: samalba)
Merge pull request #351 from dotcloud/remove-cookie

Remove cookie (flask.session)
Clean up configuration management.

This adds a default `config.yml` file which supports all options through environment variables.
You can use this to launch a container and set any parameter without having to inject your own config file.

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This also involved a few code changes.

One is to treat defined but unset config parameters as undefined. This is so that if the config references an environment variable which doesn't exist, we act as if that config param weren't set.

The other is to allow passing the elliptics node list through a string (which is what env vars are). Previously `elliptics_nodes` only accepted a hash/dictionary. This change makes it so that it also accepts a list/array or a string.
For example:

    elliptics:
      elliptics_nodes:
        - 1.1.1.1:2222
        - 1.1.1.2:2222

or

    elliptics:
      elliptics_nodes: "1.1.1.1:2222 1.1.1.2:2222"
remove old sample config files

config_sample.yml is now the comprehensive config file
update documentation for config_sample.yml changes