Resolve lookups in hook args

[ITProKyle]

closes #688

This change allows for the resolution of all stacker.lookups passed into a hook's args without needing to add a handler to each hook.

Previously, only lookups to the provided .env were resolvable or logic had to be added to hooks individually to handle lookups.

Example Usage

pre_build:
  testing-hook-lookup-resolve:
    path: hooks.test_hook.test
    args:
      rxref_lookup: ${rxref vpc::VpcId}
      nested_dict:
        xref_lookup: ${xref vpc::VpcCidrBlock}
      list_lookups:
        - ${default env_var::default_value}

Output

[2019-03-09T19:11:54] Using default AWS provider mode
[2019-03-09T19:11:54] Executing pre_build hooks: hooks.test_hook.test
[2019-03-09T19:11:54] rxref_lookup: vpc-xxxxxx
[2019-03-09T19:11:54] list_lookups: ['default_value']
[2019-03-09T19:11:54] nested_dict: {'xref_lookup': '10.0.0.0/16'}
[2019-03-09T19:11:54] ignore-this-stack: skipped (disabled)

[Spareo]

Hope this gets merged soon, this would be a super useful feature.

[phobologic]

Hey @ITProKyle - thanks for this. I think we might want to hold off on it in favor of #722 however, as that does this + adds the ability for hooks to be put directly into the execution graph, so you no longer only have pre/post build hooks. What do you think?

[ITProKyle]

@phobologic - that works. I haven't dug too deep into #722 yet to see how it's being done but as long as lookups can be resolved natively in hook args - that's all that matters.

[Spareo]

Is there something still holding this back from getting merged? This would make lookups 1000% more useful in my opinion.

[josjaf]

I'd love to see this merged as well!

[Spareo]

Any updates on this or details on what it will take to get it merged? If its not much I can try to finish it up.

[ITProKyle]

@Spareo this was paused because of a pending refactor of how hooks work that should take of this.

In the meantime, I have been added something similar to what is below to all my custom hooks which effectively does the same thing as this PR. However, it wouldn't help with any of the built-in hooks.

import logging

from stacker.variables import Variable, resolve_variables

LOGGER = logging.getLogger(__name__)

def get_variables(variables, provider, context):
    converted_variables = [
        Variable(k, v) for k, v in variables.items()
    ]
    resolve_variables(
        converted_variables, context, provider
    )
    return {v.name: v.value for v in converted_variables}

def example_hook(provider, context, **kwargs):
    """The function being called as a hook."""
    variables = get_variables(kwargs, provider, context)

    for k, v in variables.items():
        LOGGER.info('%s: %s', k, v)
    return variables