Install tar and splunk on WEC instance

[danjoneslf]

• [x] Add an exec role which is trusted to assume the forwarder role in splunk-apps/forwarders
• [x] TF in a separate statefile to avoid breaking trusts on destroy
• [x] Add an install_packages powershell script to the wec instance which:
  • [x] Assumes the role
  • [x] Downloads the /packages/* folder from S3
  • [x] Compares the file hashes of the downloads
  • [x] Runs the unpack and add to path for tar
  • [x] Runs the msi unattended for Splunk
  • [x] Validate that tar and Splunk are both installed on the WEC instance after TF apply