Merge allowlist and exclude list into ima_policy

[THS-on]

Enhancement Description

• One-line enhancement description (can be used as a release note): Merge the allowlist and exclude list into a field called ima_policy. Add a tool for converting the legacy format into the new one.
• Keylime Enhancement Proposal: (link to keylime/enhancements file, if none yet, link to PR)
• Primary contact (assignee): @THS-on
• Enhancement target (which target equals to which milestone):
  • Alpha release target (x.y)
  • Beta release target (x.y)
  • Stable release target (x.y)

Please to keep this description up to date.