Add pseudo parameter for taskcat_accountid

Hi team,

I have a usecase where I want to create S3 buckets with name that have the customer aws account id in it. Is it possible to add a pseudo parameter that will fetch the current AWS account id.

Example usage:
```
project:
   s3_regional_bucket: true
   s3_bucket: $[taskcat_accountid] // creates a bucket with name `tcat-123456789012`
```

