add support for ovf transport com.vmware.guestInfo

cloud-init OVF datasource currently supports the OVF "ISO" transport (attached cdrom).
It should be updated to also support the com.vmware.guestInfo transport.

In this transport the ovf environment file can be read with:
 vmtoolsd "--cmd=info-get guestinfo.ovfEnv"

Things to note:
a.) I recently modified ds-identify to invoke the vmtoolsd command above
in order to check the presense of the transport. It seemed to work
fine, running even before open-vm-tools.service or vgauth.service was
up. See http://paste.ubuntu.com/p/Kb9RrjnMjN/ for those changes.
I think this can be made acceptable if do so only when on vmware.

b.) You can deploy a VM like this using OVFtool and the official Ubuntu OVA files. You simply need to modify the .ovf file inside the .ova to contain
   <VirtualHardwareSection ovf:transport="iso com.vmware.guestInfo">
Having both listed will "attach" both when deployed.

c.) after doing this and getting the changes into released ubuntu
we should change the official OVA on cloud-images.ubuntu.com to have
the com.vmware.guestInfo listed as a supported transport.

Example ovftool command to deploy:
  ovftool --datastore=SpindleDisks1 \
     --name=sm-tmpl-ref \
     modified-bionic-server-cloudimg-amd64.ovf \
     "vi://<email address hidden>:$PASSWORD@10.245.200.22/Datacenter1/host/Autopilot/"