Migrate ec2inventory plugin to use boto3

[Migrated]

The upgrade to Boto3 is part of an ongoing initiative of having a single instance which coordinates deployments - with support for automatic deployments in pre-production environments.

Boto3 has built-in support for the default AWS Credentials Providers stack.

We need to set the cross-account trusting policies for our accounts, being then able to specify automatic role assumption in a cross-account context via ~/.aws/config.

It might now work out of the box, as it doesn't for aws-cli (see: https://github.com/aws/aws-cli/issues/1604 and https://github.com/aws/aws-cli/issues/1390).
In that case, we would need to do programatic role assumption - I was not been able to fully test this yet (permissions).

Note
This change is intended to be backward-compatible with the current setup - in other words, temporary/static credentials should work as before.

まず ec2inventory プラグインを見つけ、現在の AWS 認証情報の処理を追跡します。boto3 のデフォルト認証情報プロバイダーが指定されたクロスアカウント構成をサポートしているか確認し、必要であればプログラムによるロール引き受けを評価します。完了条件は、一時的/静的な認証情報との互換性を維持しながら、意図した構成を有効にできることです。

索引モデルが issue の本文から書いたものです。