upd: remove CAcert, generally improve README.md

upd: remove CAcert, generally improve README.md

Still to do, but not in this commit: extract all remaining strings like
“WPIA root” or “Test Environment” into a single file and then document
clearly in the README.md which one file should be edited or created to
put the correct information in the certificates.

Change-Id: Ic1cb0dd1fffc2bf297ab9020fa8713b9efcba868