[tf] tf change to properly configure lambda functions SA can invoke

[ryandeivert]

to @austinbyers , @jacknagz
cc @airbnb/streamalert-maintainers
size: small

bug fix

• With some updates to properly invoke a lambda function with a designated qualifier, we broke terraform's configuration of said function.
  • Previously, tf was creating a policy using the function name combined with the qualifier.
  • This prevented proper invocation with a specific qualifier using boto3
  • The change is to strip off the qualifier, and only use the function name when creating the policy.
• If a qualifier is not provided in the outputs.json configuration for this lambda, it does not break this logic. The interpolation will still extract the proper function name.
• Updating the example outputs.json configuration to include a qualifier along with the function name.

Other change

• Updating the message the lambda output configuration prompts with when asking for a function name. It previously requested an ARN, which should not be used now. It is clarified as requiring a function name and optional qualifier (function_name:qualifier)

[coveralls]

Coverage remained the same at 79.307% when pulling 43bd379 on ryandeivert-lambda-tf-fix into ebdda9c on master.

[coveralls]

Coverage remained the same at 79.307% when pulling f3c71ba on ryandeivert-lambda-tf-fix into ebdda9c on master.

[austinbyers]

Yay!

[coveralls]

Coverage remained the same at 79.307% when pulling ffbb118 on ryandeivert-lambda-tf-fix into ebdda9c on master.