BYOC: update adding-custom-cloud-credentials.md

[robert-s-lee]

• [x] add export in front of EXTERNAL_ID and ROLE_ARN
• [x] add osx install of git, terraform, jq and AWS CLI
• [x] add ubuntu / debian install of git, terraform, jq and AWS CLI
• [ ] add redhat install of git, terraform, jq and AWS CLI
• [x] add grid tool install before running grid commands in the last step
• [x] s/terraform output --json/terraform output -json/
• [ ] check instance availability

  aws ec2 describe-instance-type-offerings --location-type availability-zone  --filters Name=instance-type,Values=p3.16xlarge

[robert-s-lee]

check instance availability command below requires permissions.

aws ec2 describe-instance-type-offerings --location-type availability-zone
aws ec2 describe-instance-type-offerings --location-type availability-zone  --filters Name=instance-type,Values=p3.16xlarge
An error occurred (UnauthorizedOperation) when calling the DescribeInstanceTypeOfferings operation: You are not authorized to perform this operation.