ec2_ami: Account for AWS's "eventual consistency" with AMI creation.

Calling `ec2.get_image` right after `ec2.create_image` may raise error
"InvalidAMIID.NotFound". This has happend roughly 1 time out of 10 for me.

Other people has bitten by this too:
- https://github.com/Netflix/aminator/commit/5707f100a07d3432027d1e7a3305567a2a7d2574
- http://stackoverflow.com/a/14794952
- https://bitbucket.org/utoolity/bamboo-aws-plugin/pull-request/22/baws-116-fix-ec2-image-task-failing-with/diff