Add a preinst check that package is not being installed on platform older than where it was built on

so if a package was built on RHEL7, it should refuse to install on CentOS5.

Reason why this is so important all of a sudden:

with 3.18.0 release, we're dropping centos 5, which used for building community packages which later go to the repos. We will start putting centos 7 packages to repos.

Thus, if someone on centos5 or 6 installed CFEngine community from our repos - it will auto-upgrade to a CFEngine package which was built on a newer OS and likely won't work for them.

A way to prevent this is to add a check to the preinst script.

On a standup it was decided that best strategy for already-late 3.18.0 release is:

• for 3.18.0 release, keep all existing platforms and only add new ones (Ubuntu 20)
• Hide obsolete platforms in download tables
• after the release, remove them from Jenkins and fix what breaks.