Hi,
You can try to replace the already present certificate in the runtime/etc/keystore file by using the following commands instead of your last one (not tested, you may have to adapt the command lines):
Delete the old certificate:
keytool -delete -alias mykey -keystore /OpenHAB2InstallDir/runtime/etc/keystore
Add your own certificate:
keytool -importkeystore -srckeystore /tmp/oh.p12 -srcstoretype pkcs12 -destkeystore /OpenHAB2InstallDir/runtime/etc/keystore
-deststoretype jks -deststorepass openhab -destalias mykey
With this solution, you do not have to manipulate jetty configuration files (In OpenHAB 2, they are now embedded in a bundle).