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.

Localiza primero el plugin ec2inventory y sigue su gestión actual de las credenciales de AWS. Comprueba si el proveedor predeterminado de credenciales de boto3 admite la configuración entre cuentas indicada y, después, evalúa la asunción programática de roles si es necesario; se considera terminado cuando se conserva la compatibilidad con credenciales temporales/estáticas y se habilita la configuración prevista.

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