Use boto3 to manage credentials

[smartynoah]

The code previously used boto for delegated role credentials. This patch advances that development line to use boto3 for all credential managment.

An important effect of this change is that the plugin now works in ECS docker contexts.

[andrewegel]

I'll +1 what was mentioned here: #82 (comment)

This change is nice in that it simplifies the code, but my concerns are around pulling in a boto dependency - Limiting to distributions is one thing, but something else to bring up as a Linux Sys-Admin, is users often times pip install awscli, which may (or may not) "clobber" the system's boto, possibly changing up your boto "interface" you've coded against, thus rendering the system package manager in a slightly in-operable state.

If you compare that burden against removing ~69 lines of code thats a tough sell to me IMO.

Alternatively if this route is chosen by the maintainer or others in the community, then it would also be good to remove a bulk of the sigv2 and sigv4 methods, and rely on the underlying botocore library to do that work for you.