Splitting up iam.tf file

[eddierubeiz]

Ref #6 It became clear we will need a whole bunch of files to cover all these resources. For now I'm grouping them by function. So now we have:

• dev_users.tf
• mediaconvert.tf

The first is everything you need to know about the dev users and their user group. But split the dev mediaconvert role and policy into a separate file. They will be joined soon by staging and production counterparts.

[eddierubeiz]

To clarify:

1. The users group in dev_users.tf references aws_iam_group_policy_attachment.dev_users_mediaconvert_dev
2. Then aws_iam_policy.mediaconvert_dev (formerly in dev_users.tf and now in mediaconvert.tf) references aws_iam_role.dev_mediaconvert_role (which will be added to mediaconvert.tf in a later PR.)