allow cbc, hmac and kex to be configured individually for client and server.

This PR deprecates `require_cbc`, `weak_hmac` and `weak_kex` from the `['ssh']`attribute namespace and moves them to `['ssh']['client']` and `['ssh']['server']` instead.
The attributes in 'client' only affect the client, whereas 'server' affects only the configuration of the server.
I.e. `node['ssh']['client']['weak_hmac']` only adds hmac_sha1 to the MACs of ssh_config without weakening the hmacs of the server in sshd_config, and so forth.

Real-world usecase: gitlab server with developers using Eclipse. For EGit we have to support hmac_sha1 on the server, but I don't want to allow that for the installed client.

The PR contains compat suport for the old attributes and tests to verify the precedence logic.

The reason why I don't let the client- and server-variants take precedence over the corresponding legacy value is that an existing chef client might still have the legacy value stored, and the user may not have noticed the new attributes yet.
Instead the following logic is applied (and documented in the code):

- If one of the global legacy attributes is found with value of `true` and both client and server variants are both unaltered from their default value `false`, then the legacy attribute takes precedence and sets both to `true`.

- If either one of the client or server values is `true` we assume the user has seen the new settings and ignore the legacy value

- In any case where any of the legacy attributes is `true`, a warning is emitted, mentioning the fate of the value for both client and server configs.

__TL;DR__:
Please merge, everything will be fine ;-)