Update group_role_map_builder to pull out claim check and group list extraction into their own functions

[gene1wood]

1. Pull the check for a matching claim in an IAM Role policy out into a function which checks an element of the policy to see if it has a matching claim in it. This will allow us to change the claim name more easily
2. Pull the [process by which the an AWS IAM policy condition is turned into a group list]https://github.com/mozilla-iam/mozilla-aws-cli/blob/b1594c3dbbf97d999a3e53e66ee3ddf81806c805/cloudformation/group_role_map_builder/functions/group_role_map_builder.py#L318-L318) into it's own function. This will make it easier to change the group list format (for example from a list to a / delimited string)