Initial vcloud tool skeleton

Based largely on vcloud-net_launcher, this adds all the supporting
structure for:

* running rspec tests
* running rake
* publishing the gem
* bundler requirements
Add basic CLI tests and usage.

Ported over from vcloud-net_launcher, this adds a skeleton for
the CLI for vcloud-disk-launch.

No logic as yet, but all the CLI tests are useful and passing.
Add input config schema definition and tests

Again, ported from vcloud-net_launcher and adapted.
Basic disk creation logic

Basic disk creation logic & integration test ensure a single
disk is created successfully.

NB: This also includes an override for use of a dev version of
    vcloud-tools-tester in the Gemfile. This is due to a bug
    where network parameters and fixtures must be present, even
    though they are not required. PR to fix this is in
    https://github.com/gds-operations/vcloud-tools-tester/pull/27
Skip already-created disks

We want to be able to re-run our configuration to add new disks.

Skipping already-existing disks is the simplest way of handling
this behaviour.

In the process, add some logging to give the user feedback.
Update vcloud-core, for IndependentDisk#destroy
Use vcloud-tools-tester v0.3.0

Prior versions required that fixture networks be set up for the higher level
vCloud entities (eg vApps, edgeGateways). We do not need networks for any
of our tests, so this update prevents us having to specify and create them.
Add integration test for disk_launcher

Extremely simple -- just test that YAML reading is working, and that
the calls to vCloud Director via vCloud Core is as expected.

All we need to do this is add a single small disk.

Naturally, clean up after ourselves.
Update README

NB: this is taken largely from vcloud-launcher and adapted
Add a CHANGELOG
Merge branch 'master' into initial_working_version

Conflicts:
	README.md
Merge pull request #1 from ministryofjustice/initial_working_version

Initial working version