Add missing instance size and normalization factor

[chaspy]

Found the following error.

2019-08-24 12:40:11 UTC | ERROR | dd.collector | checks.aws_ec2_count(__init__.py:841) | Check 'aws_ec2_count' instance #0 failed
Traceback (most recent call last):
  File "/opt/datadog-agent/agent/checks/__init__.py", line 824, in run
    self.check(copy.deepcopy(instance))
  File "/etc/dd-agent/checks.d/aws_ec2_count.py", line 318, in check
    reserved_instances = fetcher.get_reserved_instances()
  File "/etc/dd-agent/checks.d/aws_ec2_count.py", line 246, in get_reserved_instances
    reserved_instance['InstanceType'],
  File "/etc/dd-agent/checks.d/aws_ec2_count.py", line 132, in get_itype
    return self.get(az, family, size)
  File "/etc/dd-agent/checks.d/aws_ec2_count.py", line 126, in get
    self.__instances[az][family][size] = InstanceCounter(NormalizationFactor.get_value(size))
  File "/etc/dd-agent/checks.d/aws_ec2_count.py", line 31, in get_value
    raise TypeError('unknown instance size : {}'.format(size))
TypeError: unknown instance size : 12xlarge

[chaspy]

Hi, @mounemoi
Thank you for the very useful software!
After we start using 12xlarge class, the agent didn't work.

If I should write more test, I would be happy if you could guide me 🙏
Please review and merge.

[chaspy]

I applied these changes to our agent's configuration, it works fine 👍

[isaoshimizu]

On behalf of @mounemoi, there was no problem with review and operation check. Thank you for the pull request.