feat(execute.go): add flag specification for custom actions

[vdice]

Adds a flags section that can be added/used for all custom actions (as the flag list for a given terraform command may differ, we need a way to supply arbitrary values).

Closes #24
Depends on getporter/porter#681

[carolynvs]

Seems like terraform supports both - and -- for its flags? Is that not working?

[vdice]

@carolynvs Indeed, it seems with most flags/options, -- is supported... but I had stumbled on --no-color, as one example where it didn't appear to work:

 $ terraform plan --no-color
flag provided but not defined: -no-color
Usage: terraform plan [options] [DIR-OR-PLAN]

  Generates an execution plan for Terraform.

  This execution plan can be reviewed prior to running apply to get a
  sense for what Terraform will do. Optionally, the plan can be saved to
  a Terraform plan file, and apply can take this plan file to execute
  this plan exactly.

  If a saved plan is passed as an argument, this command will output
  the saved plan contents. It will not modify the given plan.

Options:
...
  -no-color           If specified, output won't contain any color.
...

If it turned out that was really the only place, we/I should probably file an issue in the terraform cli... meanwhile, I figured it'd be safer to go the single - route as shown in the help text, etc.

[carolynvs]

Gotcha! I had seen terraform listen to some of my flags in the past and didn't realize that was just luck. 😀