allow get_filter_value to return state of ami and test

Moto didn't support filtering AMIs by their 'state' value.  This PR enhances moto to allow state filtering on AMIs.

  * updated Ami.get_filter_value to return the state value of the AMI
  * updated test_ec2.test_amis.test_ami_filters to test for AMIs in the state of 'available'

This was done per: http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-DescribeImages.html