Customize ssh_timeout and ssh_retries

This pull request is related to allowing a user to customize the `sleep` and `retries` mentioned in @sethvargo's open issue: https://github.com/test-kitchen/test-kitchen/issues/422. The defaults given were what the code currently has:
```ruby
ssh_timeout: 1
ssh_retries: 3
```
and neither are required to be set.

To customize, a `.kitchen.yml` would look like:
```yml
---
driver_plugin: ec2
driver_config:
  ssh_timeout: 15
  ssh_retries: 5
  aws_access_key_id: 'YOUR VALUE'
  aws_secret_access_key: 'YOUR VALUE'
  availability_zone: "us-east-1a"
...
more config here
...
```