island: Add `umask 377` to create_certificate.sh

By setting the umask to 377 in create_certificate.sh, we ensure that the
TLS key file that is created is readable only by the user that created
it, and not world readable (as is the default on some linux distros).

monkey/monkey_island/linux/create_certificate.sh

@@ -17,6 +17,8 @@ if [ ! -f /tmp/foo.txt ]; then  # If the file already exists, assume that the co
   CREATED_RND_FILE=true
 fi
 
+umask 377
+
 echo "Generating key in $server_root/server.key..."
 openssl genrsa -out "$server_root"/server.key 2048
 echo "Generating csr in $server_root/server.csr..."