Use ARN resolver on assumeRoleArn.

[derrickburns]

Addresses #271

[cyrus-mc]

@derrickburns

Great PR. I was just about to do this myself but once again the community has already provided. I built and tested and it works great.

[derrickburns]

Cool! I need to actually use it. Any ETA on a release with it?

[Joseph-Irving]

Hi, thanks for this!

The test are failing with

# github.com/uswitch/kiam/pkg/server
pkg/server/server.go:113:22: undefined: v1
pkg/server/server.go:120:22: undefined: v1
pkg/server/server.go:307:12: undefined: v1

[derrickburns]

I submitted a new PR. I ran the tests. They pass.