Drop Ubuntu 16.04, add 18.04

[damonbarry]

This change updates our YAML pipelines for the 1.1 release to generate and consume bits built on/for Ubuntu 18.04 instead of 16.04. Specifically:

• Connectivity, End-to-end, Long Haul and Stress pipelines + downstream scripts: replace references to Ubuntu 16.04 artifacts with 18.04
• Edgelet Packages pipeline + downstream scripts: don't build Ubuntu 16.04 artifacts
• Remove Ubuntu 16.04 cross-compile scripts
• Various markdown files, tests, comments, and schema examples: change Ubuntu 16.04 references to 18.04

Note: this change does not update the arm-based docker containers that the K8s betas use for iotedged. They're still 16.04-based in this branch, and will be updated later.

[arsing]

If you're dropping Ubuntu 16.04 support, you should drop Debian 8 support too. (Should be just in package.sh and any YAMLs that reference it.)

[damonbarry]

If you're dropping Ubuntu 16.04 support, you should drop Debian 8 support too. (Should be just in package.sh and any YAMLs that reference it.)

Yeah, and Rasbian Stretch (which exited LTS around the same time as Debian 8). But I plan to handle those separately...just focused on Ubuntu in this PR.