Add env variable flexibility to raem account setup

[johnclaus]

I've found that AMAZON_ACCESS_KEY_ID/AWS_ACCESS_KEY and AMAZON_SECRET_ACCESS_KEY/AWS_SECRET_KEY are used interchangeably throughout AWS docs and comparable tools.

Since these seem to be environment variables used throughout these toolchains, I thought it might be useful to add some flexibility to raem while creating a new account. This is a change I've used locally for a while, but I always forget to push it upstream.

[mheffner]

Should these also be set in the list of environs that are exported from awsam when switching accounts? If so, want to open a followup PR to add those?

[mheffner]

Merged, thanks!