support multiple aws credentials

[yarikoptic]

for different projects/resources users might need to use different aws credentials. ATM AFAIK (but I could be wrong) we support only a single [aws] section within ~/.config/reproman/reproman.cfg. We should allow support for choosing which "credentials pair" to use for a specific new resource. ATM we seems to support specifying keys explicitly:

$> reproman backend-parameters
Backend parameters for 'aws-ec2'
  access_key_id: AWS access key for remote access to your Amazon subscription.
...  secret_access_key: AWS secret access key for remote access to your Amazon subscription
...

but we don't want to force specifying them on cmdline, thus pointing to specific aws credential pair would be useful, and we might like to store that name within our ec2 resource inventory spec.