Implement support for tests using real libvirt with FOG_LIBVIRT_URI

[davispuh]

Currently rake live fails with:

Fog::Compute[:libvirt] (libvirt)     
    fog-libvirt/tests/libvirt/compute_tests.rb
      Fog::Compute[:libvirt] (libvirt)
    Missing required arguments: libvirt_uri (ArgumentError)

First commit from this PR fixes that so rake live works.
2nd commit implements support for running tests against real libvirt by using FOG_LIBVIRT_URI env var.
All these are fully supported and work correctly:

$ rake
$ FOG_MOCK=true rake

$ FOG_MOCK=true FOG_LIBVIRT_URI=qemu:///session rake
$ FOG_MOCK=true FOG_LIBVIRT_URI=qemu:///system rake

$ FOG_MOCK=false rake
$ rake live

$ rake live_session
$ FOG_LIBVIRT_URI=qemu:///session rake
$ FOG_MOCK=false FOG_LIBVIRT_URI=qemu:///session rake
$ FOG_LIBVIRT_URI=qemu:///system rake
$ FOG_MOCK=false FOG_LIBVIRT_URI=qemu:///system rake

(something like FOG_LIBVIRT_URI=qemu+ssh://host/system rake should also work but I haven't tested)

I also fixed tests so they cleanup afterwards so there won't be test leftovers when running against real libvirt.
But note that with this feature tests shouldn't do something like servers.all.each(&:destroy) to not cause someone a bad day when run with real libvirt. If needed then such should only be run when using libvirt test driver only.

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[ekohl]

Thanks, looks really good. Only some minor inline comments but I'd be happy to merge this as is already.

[davispuh]

Just pushed with all fixes applied.

[ekohl]

Thanks!