Add hw/vm tags, add single link tag.

Add HW_EVN and VM_ENV tags to specify limitations on where the given
test could be run. For tests that should be run only on bare metal,
author of the test should add tag HW_ENV to it.

Add 3_NODE_SINGLE_LINK_TOPO tag, to specify requirement on topology,
where only one link is required in between any two nodes in the
topology.

Change-Id: I38c9a9732cad608bb6a9e4a2c5f1997236cd48b9
Signed-off-by: Stefan Kobza <skobza@cisco.com>