Minor cleanup and refactoring of ec2-fetch-ssh-key-fingerprint

Increased the sleep to 20 seconds, as it takes a while for the
console output to be available. Also, corrected the path to
call ssh-add-known-host.
Added reverse DNS lookup to ssh-add-known-host script

If the user provides a hostname the script will now
use `host` to do a reverse DNS lookup and get the IP
address. When the SSH public key is added to known_hosts
both the hostname and IP address are included when
available.

Also, the ssh-keyscan -H option is no longer used to hash
the public key, as it made removing existing entries in
the known_hosts file unpredictable.
Make sure that $ip_address only returns a single IP
Cleanup and new task to add instance's SSH public key to known_hosts

Updated the `image_id` to latest Ubuntu Server AMI in us-east-1 region.
Also, refactored tasks to use instance's DNS name instead of IP address.

Replaced non-working tasks with a new task to add the instance's
SSH public key to known_hosts without user intervention. This was
possible with the ec2-fetch-ssh-key-fingerprint and ssh-add-known-host
scripts.
We don't need to bootstrap or mess with /etc/sudoers
Added task to remove instance's SSH public key from known_hosts
Now running and testing the playbooks