Added AWS Instances Cloudwatch collector

Please review the code and let me know what all changes we can do to improve it. It collects datapoints exposed by AWS cloudwatch for all the instances running in all AWS regions.

Sample output:
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aws.networkin 1387282020 174.0 instance-id=i-f5cb42c1 statistic=Average placement=us-west-2c
aws.networkin 1387282020 174.0 instance-id=i-f5cb42c1 statistic=Minimum placement=us-west-2c
aws.networkin 1387282020 174.0 instance-id=i-f5cb42c1 statistic=Maximum placement=us-west-2c
aws.diskwriteops 1387282020 0.0 instance-id=i-f5cb42c1 statistic=Average placement=us-west-2c
aws.diskwriteops 1387282020 0.0 instance-id=i-f5cb42c1 statistic=Minimum placement=us-west-2c
aws.diskwriteops 1387282020 0.0 instance-id=i-f5cb42c1 statistic=Maximum placement=us-west-2c
aws.networkout 1387282020 132.0 instance-id=i-f5cb42c1 statistic=Average placement=us-west-2c
aws.networkout 1387282020 132.0 instance-id=i-f5cb42c1 statistic=Minimum placement=us-west-2c
aws.networkout 1387282020 132.0 instance-id=i-f5cb42c1 statistic=Maximum placement=us-west-2c
aws.diskwritebytes 1387282020 0.0 instance-id=i-f5cb42c1 statistic=Average placement=us-west-2c
aws.diskwritebytes 1387282020 0.0 instance-id=i-f5cb42c1 statistic=Minimum placement=us-west-2c
aws.diskwritebytes 1387282020 0.0 instance-id=i-f5cb42c1 statistic=Maximum placement=us-west-2c