Show plan before executing it; add --dry-run option to just show plan and exit

[nchammas]

Not sure how useful this would be, but one feature I've seen elsewhere that might make sense in Flintrock is to have some concept of an "execution plan".

Just as Flintrock is about to do something, it lays out its plan. This makes it clear what options are taking effect, which could be helpful when you have a config file and command-line options to resolve. This would just be some text shown at the start of any operation.

For example:

$ ./flintrock launch shooper-dooper-clushter
Using config file: /path/to/flintrock/config.yaml
Launching:
 - 1 master, 2 slaves
 - Spark installed, HDFS not installed
 - us-east-1 region

This would fit well with the changes proposed in #27.

An additional idea would be to have some kind of --dry-run option which just shows the plan and exits.