Fixes `--secret` and `--secret-file` options

* propagates knife.rb configurations
* adds CLI options to `knife ec2 create` and prefers those values over
what is in knife.rb
* adds specs
Merge pull request #145 from opscode/adamed-knife-349

KNIFE-349: Propagate "secret" and "secret-file" values for use when bootstrapping a new EC2 instance