Credentials fail when aws.ec2metadata is not available

[leeper]

From 9627d8f

When running from 0.3.4 I get this error:

Loading required namespace: aws.ec2metadata
Failed with error:  'there is no package called 'aws.ec2metadata''
Error in locate_credentials(key = key, secret = secret, session_token = session_token,  :
  object 'cred' not found
Calls: source ... eval -> <Anonymous> -> s3HTTP -> locate_credentials
Execution halted

We reverted to the 0.3.3 version found on CRAN and that solved it.

If we install aws.ec2metadata the error becomes:

Loading required namespace: aws.ec2metadata
Error in curl_fetch_memory(uri) : Couldn't connect to server
Error in curl_fetch_memory(uri) :
  URL using bad/illegal format or missing URL
Error in role[["AWS_ACCESS_KEY_ID"]] : subscript out of bounds
Calls: source ... eval -> <Anonymous> -> s3HTTP -> locate_credentials
Execution halted

[danielerapati]

Thank you for opening this issue.

A key information is that we don't run from an EC2 and pass the credentials as environment variables.

[willbowditch]

Problem appears to be that role <- try(get_ec2_role(verbose = verbose), silent = TRUE) returns

[1] "AccessKeyId"     "Code"            "Expiration"      "LastUpdated"     "SecretAccessKey"
[6] "Token"           "Type" 

but locate_credentials() expects similar names but with underscores:

if (!inherits(role, "try-error")) {
      if (!is.null(role[["AWS_ACCESS_KEY_ID"]])) {
        key <- role[["AWS_ACCESS_KEY_ID"]]
        if (isTRUE(verbose)) {
          message("Using EC2 Instance Metadata for AWS Access Key ID")
        }
      }
      if (!is.null(role[["AWS_SECRET_ACCESS_KEY"]])) {
        secret <- role[["AWS_SECRET_ACCESS_KEY"]]
        if (isTRUE(verbose)) {
          message("Using EC2 Instance Metadata for AWS Secret Access Key")
        }
      }
      if (!is.null(role[["AWS_SESSION_TOKEN"]])) {
        session_token <- role[["AWS_SESSION_TOKEN"]]
        if (isTRUE(verbose)) {
          message("Using EC2 Instance Metadata for AWS Session Token")
        }
      }
      if (!is.null(role[["AWS_DEFAULT_REGION"]])) {
        region <- role[["AWS_DEFAULT_REGION"]]
        if (isTRUE(verbose)) {
          message(sprintf("Using EC2 Instance Metadata for AWS Region ('%s')", 
            region))
        }
      }

[leeper]

Folding this into #142