CA-145878: make the hotfix signing key configurable

The xapi.conf file can override the hotfix signing key fingerprint
by:

  hotfix-fingerprint=<fingerprint>

This extends the FIST-point activated hotfix testing key mechanism
that we already have.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #1936 from djs55/change-hotfix-key

CA-145878: make the hotfix signing key configurable