move IAM service configuration to a separate config file

[benmcclelland]

Describe the solution you'd like
The IAM service options are getting a bit overwhelming in the help output. Since we anticipate several more IAM service integrations in the future, lets move the IAM service configuration out of the cli args and into a separate config file.

The goal here is to reduce the cli options to just the following:

--iam-dir
--iam-config

We can leave the internal IAM service support with --iam-dir option for easier testing, but all others should be configured through a config file specified with --iam-config

[benmcclelland]

It looks like TOML might be a good format for intuitive config file management: https://godocs.io/github.com/BurntSushi/toml

[benmcclelland]

We may also want to consider that some options could be set by environment variables as well.