* Adding support for Amazon ECR
This patch adds a new module named ecr, which can create, update or
destroy Amazon EC2 Container Registries. It also handles the management
of ECR policies.
* ecs_ecr: addressed review feeback
* Renaming ecr to ecs_ecr
* Fixed docs
* Removed bad doc about empty string handling
* Added example of `delete_policy`
* Removed `policy_text` option; switched policy to `json` type so
it can accept string or dict
* Added support for specifying registry_id
* Added explicit else after returned if clauses
* Added `force_set_policy` option
* Improved `set_repository_policy` error handling
* Fixed policy comparisons when AWS doesn't keep the ordering stable
* Moved `boto_exception` into the module
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