Add storm-webdav's cert creation to the rpm and deb packages

[vrbanecd]

Create the initial storm-webdav certificate in the RPM/DEB packaging

The CA-signed certificate for storm-webdav currently has to be created by hand
after installing the package. That should happen automatically as part of the
install.

Add to the postinstall step of both the RPM and DEB packaging:

• Generate the initial certificate if none is present yet (don't overwrite an
  existing one on upgrade)
• Set correct ownership and permissions on the key
• Install the renewal cron job file at the same time

The renewal must be shipped as a cron job file (drop-in under /etc/cron.d/) and
be part of the package, not something added by hand with crontab -e.