ssh keys configuration for virtual machine [#8]

This is the proposed solution:

1. node['teracy-dev']['ssh'] configuration is used, default: `false`. It means that no ssh keys needed for the virtual machine.

2. node['teracy-dev']['ssh'] must be changed to `true` to use ssh keys.
  + To use existing keys, copy both public and private keys with name `id_rsa` and `id_rsa.pub` into `cookbooks/teracy-dev/files/default`.
  + If you don't copy, then the first of ssh login into the virtual machine, a prompt will ask you to enter email and generate ssh keys. After keys are generated, they are copied into `cookbooks/teracy-dev/files/default` for later use.