Update the libvirt configuration.

Management access to libvirt is controlled through membership to a unix
group, on fedora/rhel we need to updated the libvirt configuration to
give management access to the libvirtd group.
Add -e (engine) option to boot-elements script.

By default engine is set to kvm.

The motivation is to be able to use this to perform nested
VM CI using the upstream infrastructure.
Merge pull request #30 from dprince/boot-elements-engine

Add -e (engine) option to boot-elements script.
Merge pull request #28 from umago/update-libvirtd-conf

Update the libvirt configuration.
Add python-dev and gcc dependencies for Debian
Merge pull request #32 from pleia2/master

Add python-dev and gcc dependencies for Debian
Remove restart of libvirt-bin from notes.md

Restarting of libvirt-bin is now included in
scripts/install-dependencies so it's not necessary to
repeat it.
Reworking docs.
Adds qemu-img to list of required Red Hat packages

We need this to provide qemu-nbd.
Add qemu-utils for Debian/Ubuntu.

We need this for qemu-nbd.
Merge pull request #33 from pleia2/master

Remove restart of libvirt-bin from notes.md
Merge pull request #34 from dprince/install-deps-qemu-img

Adds qemu-img/qemu-utils to list of required packages.
More doc improvements.
Update architecture section.
Finish review of README.md.
Add squid cache instructions

Add steps to enable squid cache proxy to speed up image build times.
Merge pull request #36 from jogo/squid

Add squid cache instructions
First pass overhaul of devtest.md.
Stop installing tools globally.
Update d-i-c invocation to modern usage.