Add short server names to Debian Preseed server

Before you could direct Debian Installer to a preseed server using long
hostnames like 'debian.nat.exmple.com' to a normal preseed and
'destroy.debian.nat.example.com' to a destructive preseed. Now, Ansible
will configure nginx server to also accept shortened hostnames, like
'debian' and 'destroy.debian'. These hostnames need to be configured
separately in your DNS to point to the correct server.
Update plays in virtualization.yml playbook

KVM and LXC plays were using old variables of roles 'nat' and 'dnsmasq',
they have been updated to use new ones. Tags in 'kvm' role has been
moved to virtualization.yml playbook.
Add a variable with libvirt group to 'kvm' role

System group that manages libvirt is different on Debian and Ubuntu. To
allow easier management, you can set it using 'kvm_group' variable in
inventory.
Update Debian Netinst URL to Debian 7.5.0
Reconfigure network interfaces only if required

When network-manager is present, list of interfaces to reconfigure might
be empty. This change will allow reconfiguration task to be skipped
without issues.
Add variables for custom preseed domain in 'apt'
Change default Debian Preseed hostname on Travis
Merge pull request #64 from drybjed/bugfixes

Bugfixes in multiple roles