Ignore failure flag when ami datasource can't find ami

[Gal Birkman]

Description

I would like to add an option for ami datasource that will "ignore failure" when a ami is not found between aws ami's.

Use Case(s)

I have 2 cases using same packer file:

• the ami exists
• the ami does not exits, and I want to fall back to hardcoded ami id.
  The current version of the plugin fails when the datasource doesn't find any ami by the given filters.

Potential References

Error example:
Error: Datasource.Execute failed: No AMI was found matching filters:

I will apply a PR for this one and mention it here.