Set container utsname to configured hostname per default

Previously, the utsname of a machine was set to the vagrant machine ID.
As the utsname represents the hostname of the machine and is independent
from the name of the actual LXC container, the hostname that was
specified in the Vagrantfile should be used instead. If no hostname is
provided, the machine ID will be used like before.

Additionally, this will trigger the DHCP client in the container to send
the correct hostname to the DHCP server on the first request at boot.
Vagrant sets the configured hostname only after the network is up. If
for example automatic DNS updates are configured, the right DNS record
will be created every time.