Demo server certificate install path differs from Yocto to deb package

In Yocto builds, the server is installed at /etc/mender/server.crt, while the deb package installs it at /usr/share/doc/mender-client/examples/demo.crt (following Debian guidelines).

This makes mender setup command to not be able to properly set a demo environment for Yocto builds (see related issue MEN-3047).

At the moment of writing this ticket, is it a bit uncertain what should be done; the easiest will be to use the same path in both installations and release it in next yocto version zeus, but still we need to make it work in current warrior branch and upcoming mender 2.3.

Acceptance criteria:

• Yocto recipe for mender installs the demo certificate in /usr/share/doc/mender-client/examples/demo.crt
• Acceptance tests and scripts are updated accordingly