Added get-dns-from-instance-id and get-instance-id-from-dns to do the…

[tchia04]

… conversion without using console or complicated cli

Sample Usage:

aws get-dns-from-instance-id i-aabbccddeeffgghh

aws get-instance-id-from-dns foo.bar.example.com

[kyleknap]

Thanks for the PR! One question I have is if you possibly simplify both to:

get-dns-from-instance-id =
  !f() {
    aws ec2 describe-instances \
    --instance-ids "${1}" \
    --query 'Reservations[*].Instances[*][PublicDnsName]' --output text
  }; f

get-instance-id-from-dns =
  !f() {
    aws ec2 describe-instances \
    --filters "Name=dns-name,Values=${1}" \
    --query 'Reservations[*].Instances[*][InstanceId]' --output text
  }; f

That way you do not have to pipe the output to cut. As a bonus, if you do not want to run this in a separate subprocess, you could simplify one of the aliases to:

get-dns-from-instance-id =
    ec2 describe-instances \
    --query 'Reservations[*].Instances[*][PublicDnsName]' --output text \
    --instance-ids