Make the log group optional

[djmcgreal-cc]

Is your request related to a problem? Please describe.

My IAM credentials do not have the necessary permissions to read/write log groups. This module has two modes:

• with use_existing_use_existing_cloudwatch_log_group=false is the default and tries to create log group resources
• with use_existing_cloudwatch_log_group=true it tries to read them.

I'm able to create the lambda by giving it a lambda_role that can create its own log group.

Describe the solution you'd like.

Addition of an input that neither reads nor writes log groups.