ssh keys configuration for virtual machine [#8]

This is the proposed solution:

1. To use ssh keys on the virtual machine, you must configure the use of ssh from
``Vagrantfile``: replace ``"ssh" => false`` by ``"ssh" => true``.

2. Use existing ssh keys: ``id_rsa`` and ``id_rsa.pub`` from *cookbooks/teracy-dev/files/default*
will be always copied into the virtual machine each time of login into the virtual machine. This
mechanism is used to make sure you can always have updated ssh keys into the virtual machine. Just
put files there, and you're done after ``$ vagrant ssh`` again.

3. If you don't want to use existing ssh keys (means that no id_rsa and id_rsa.pub in the cookbook),
then ``$ vagrant ssh`` will ask you to create ssh keys right after login. You need to use default
key name (id_rsa). These are new generated keys. So to use for ssh access, you must provide this
new public key to ssh servers (add public key to github, bitbucket accounts).
These new generated keys will be also copied into *cookbooks/teracy-dev/files/default*. When the
virtual machine is ``destroy`` and ``up`` again, it will be copied into ``./ssh`` directory of the
virtual machine again as described in step 2 above.