Use latest Ubuntu AMI as base for our AMIs

Closes: #5145 (closed)

Development log:

• Use the latest (as of now, the one released in 2020-05-21) Ubuntu AMI (hvm, ebs-ssd)
• Run apt-get upgrade as part of build, so that any new security fixes from Ubuntu gets installed
• Dynamically choose the latest Ubuntu AMI as base, using ami_source_filter directive.
  • Pros:
    • We don't have to remember to update this every now and then.
  • Cons:
    • Finding out what is the source AMI used requires reading the logs. However, I don't remember a situation when we had to find out that information, ever since AMIs were first introduced.