Set container utsname to its actual hostname

Previously, the utsname of a machine was set to the vagrant machine ID.
As the utsname represents the hostname of the machine and is independant
from the name of the actual LXC container, the hostname that was
specified in the Vagrantfile should be used instead.

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