Support AWS instance profiles

[Howard Butler]

As described in PDAL/PDAL#1448

[Michael D. Smith]

So the method for getting this information, if not using the SDKs, is to use curl to query the instance metadata to get the credentials

1. Get the IAM role available from the instance:
   curl http://169.254.169.254/latest/meta-data/iam/security-credentials/

2. Get the credentials from the call to that role
   curl http://169.254.169.254/latest/meta-data/iam/security-credentials/<role_from_step1>

This returns JSON of the temp security credentials:

{
  "Code" : "Success",
  "LastUpdated" : "2016-12-18T16:04:09Z",
  "Type" : "AWS-HMAC",
  "AccessKeyId" : "<access_key_id>",
  "SecretAccessKey" : "<secret_access_key>",
  "Token" : "<token>",
  "Expiration" : "2016-12-18T22:38:19Z"
}

3. Then use these credentials to access as outlined here (http://docs.aws.amazon.com/IAM/latest/UserGuide/id_credentials_temp_use-resources.html)

I think a new expiration date in generated every 15 min