Use debian_gem for bootstrapping Debian Wheezy

The Omnibus installer does not support Debian 7.0 Wheezy yet.
Drop support for Debian 5.0 "Lenny"

It reached EOL already one year ago.
Extract debianoid codename fetching to a method
Use gem installation method for all unknown Debian versions

This covers all unreleased versions.
Use ternary operator instead of one-lines if-else statements
Clean up bootstrap tests
Append --omnibus-version to --omnibus-options

Also add unit test to verify that the specified options will
be passed to the Omnibus installer.
Rename Cook command's `--skip-chef-check` option to `--no-chef-check`

Knife convention is to use "--no-option" or "--[no-]option". In that
case the "Mixlib::CLI" class allows the option with or without the "no".
And this gives also the opportunity to set the default on knife
configuration and then override it on command line.

The old "skip" option is left with a deprecation warning for some period
of time to maintain backwards compatibility.
Remove singleton instance variable for #operating_system

The method is only called once.
Get Chef version from `chef-solo --version`

Verify the version ourself with Get::Requirement.
Merge pull request #173 from tmatilai/omnibus_options

Append --omnibus-version to --omnibus-options
Merge pull request #172 from tmatilai/wheezy

Pepare fails on Debian 7.0 wheezy
Debian 6 integration test
Fix class name
Adding Debian 7 support
Update the Changelog
Upgrade Chef dev dependency to v10.18.0

mixlib-cli v1.3.0
Merge pull request #162 from tmatilai/chef-check

Rename Cook command's `--skip-chef-check` option to `--no-chef-check`
Fix parallel intgration tests using same AMI

Use unique name for the server based on the test class name
instead of AMI.
Merge pull request #168 from tmatilai/fix-parallel-ec2-tests

Issues running both ubuntu12 integration tests at the same time