Default value of 'sudo' set to False.
Runner param can be defined immutable in action

* A runner param can be declared immutable in action. In this case,
we can use the 'default' value supplied in either runner or action.
We need default to be specified in at least one place.
Fix action validation regarding immutability and default values

* Default value for a runner param made immutable in either
runner or action can also come from either.
Fix packs in st2 repo.
Add test cases
Merge pull request #937 from StackStorm/fix_st2_packs_actions

Fix st2 packs actions