Add network debugging output
Remove one branch that can never be taken and comment another that can only be taken in older vagrant
Create default network if needed. Relates to #102.

This commit

* renames the option default_network to default_network_name
* introduces the option default_network_address, since we need an
address in order to create a network
* handles creation of the default network similarly to other private
networks if needed
Fail if any existing networks configured to have DHCP do not have it. Relates to #103 and #123.
Make network creation thread-safe, closes #104
Fix logic for debug output about finding network by ip
Use vagrant-libvirt and 192.168.121.1/24 as default network settings.
Closes #125.
Rename default network to management network. Closes #126.

The use of the term default makes discussing this network confusing.
This network must exist and be configured a certain way in order for vagrant
to manage a VMs configuration, so lets call it the management network instead.
Merge branch 'improve_network_robustness' of github.com:sciurus/vagrant-libvirt into sciurus-improve_network_robustness

Conflicts:
	README.md
	lib/vagrant-libvirt/config.rb