Updated plan to use an instruction mapping instead of an if statement

This is the next step in #997 

cc @asmeurer @ilanschnell 
Sorry, the traceback I provided was actually `conda env remove`, here's `conda env remove --all`:

```
Traceback (most recent call last):
  File "/Users/travis/anaconda/bin/conda", line 7, in <module>
    execfile(__file__)
  File "/Users/travis/Dropbox/work/github/conda/conda/bin/conda", line 5, in <module>
    sys.exit(main())
  File "/Users/travis/Dropbox/work/github/conda/conda/conda/cli/main.py", line 202, in main
    args_func(args, p)
  File "/Users/travis/Dropbox/work/github/conda/conda/conda/cli/main.py", line 207, in args_func
    args.func(args, p)
  File "/Users/travis/Dropbox/work/github/conda/conda/conda/cli/main_remove.py", line 130, in execute
    actions = {plan.PREFIX: prefix,
AttributeError: 'module' object has no attribute 'PREFIX'
```

I'm about to push a fix.