[knife-ec2]Command-line options do not take precedence over knife.rb configuration (#247)

@adamedx @muktaa
Issue ref: https://github.com/opscode/knife-ec2/issues/247

Actually issue is with chef where in bootstrap flow knife.rb has precedence over CLI options.
So added changes to support standard precedence level(i.e CLI override knife.rb).
Okay, @sersut convinced me there's a way to get some value from keeping these tests. If we remove the knife.config lines from the let, and then add `knife.merge_configs` as the next to last line of the let, then this block of tests would ensure that #knife_ssh does in fact correctly set the ssh hash from config values passed to it (it does not test precedence, as we've discussed previously, that's merge_configs job and hard/impossible to test as a unit test).