Generate self-signed cert in /etc/ssl/private/kasmvnc.pem

I piggyback on ssl-cert package that's got ssl-cert group and the
directory /etc/ssl/private, the group can read files from (but not list
files there). Thus, by adding a user to ssl-cert group, they can read
both ssl-cert certificates and the KasmVNC certificate.o

Note: currently, KasmVNC only supports one file that must contain both
private and public keys. For this reason, I didn't use the snakeoil
certificate from ssl-cert, as it's split into two files.

debian/control

@@ -10,7 +10,7 @@ Homepage: https://github.com/kasmtech/KasmVNC
 
 Package: kasmvncserver
 Architecture: amd64
-Depends: ${shlibs:Depends}, ${misc:Depends}, ${perl:Depends}, python, xauth
+Depends: ${shlibs:Depends}, ${misc:Depends}, ${perl:Depends}, ssl-cert, python, xauth
 Provides: vnc-server
 Description: virtual network computing server with web interface
  Lorem ipsum