Step: vgcreate
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Create an LVM2 volume group (VG), and also initialise the physical
volumes for it.


Step keys:

* `vgcreate` &mdash; REQUIRED; value is the tag for the volume group.
  This gets initialised with `vgcreate`.

* `physical` &mdash; REQUIRED; list of tags for block devices
  (partitions) to use as physical volumes. These get initialised with
  `pvcreate`.

Example (in the .vmdb file):

    - vgcreate: rootvg
      physical:
      - my_partition
      - other_partition
