Allow Elastigroup component to use custom Taupage AMI

[Luis]

Currently, Elastigroup components always use the latest production Taupage AMI.

When the attribute Image is specified that setting should be honored, allowing users to easily specify one of the other existing options - LatestTaupageStagingImage or LatestTaupageDevImage

The current workaround is to customize use the Elastigroup attribute:

Elastigroup:
  compute:
    launchSpecification:
      imageId: <ami-id>

Which requires users to know in advance the AMI ID and hardcode it.