Add support for iam_role in service_shard_update

[panbenson]

create a --iam-role option to provide permissions when using service_shard_update

[nemacysts]

@panbenson just curious: how would this be used? would a human manually add something like

my_cool_config_section: &my_cool_anchor
    ...

to the file and then call this script with --anchor my_cool_anchor?

[panbenson]

@panbenson just curious: how would this be used? would a human manually add something like

my_cool_config_section: &my_cool_anchor
    ...

to the file and then call this script with --anchor my_cool_anchor?

@nemacysts yes that's the idea! that way additional/custom config can be added for sections once they've gone thru review

edit: switch this to configure iam role only!