Add legacy support for Trusty cloud package

[lucasmoura]

Why is this needed?

We are adding the same change we did on #3049 to the package that is being used on the Trusty Pro cloud images.

Test Steps

1. Launch an Azure Pro Trusty image
2. Build the package and install it on the image
3. See that esm-infra-legacy doesn't appear on status
4. Detach the machine
5. Run ua status and check that esm-infra-legacy appears on the output
6. Change the contract url to staging
7. Attach the machine with the staging token entitled to esm-infra-legacy
8. Check that esm-infra is enabled, but not esm-infra-legacy
9. Enable esm-infra-legacy and see that it appears as enabled on ua status
10. Disable the esm-infra-legacy and check that esm-infra is still enabled

Checklist

• [x] I have updated or added any unit tests accordingly
• [ ] I have updated or added any integration tests accordingly
• [ ] Changes here need to be documented, and this was done in:

Does this PR require extra reviews?

• [ ] Yes
• [x] No

[lucasmoura]

Closing this PR as we already have a PPA with the package