net-mgmt/icingaweb2-module-vspheredb: allow flags in rc.d script

Add vspheredb_flags to rc.d script allowing users to choose which
daemon(8) they wish to use. My use case is:

vspheredb_flags="-o /var/log/vspheredb.log"

Which logs the output of the daemon to the specified file.

See man(8) daemon for more information.

Approved by: maintainer (via private communication)