Various customisations; add $GUESTNAME as parameter to dhcp client request.

* Use sh instead of bash as bash not installed in boot2docker.
* Provide the value of $GUESTNAME as an argument to the DHCP client so the DHCP server can add it to DNS (if the DHCP server is also acting as a DNS server, e.g. dnsmasq).
s/maybe/may be/