[tempest]: Make public network name configurable.

[mmnelemane]

Tempest barclamp expects the public network name to be "floating" and is hard coded in the recipe. Some special use-cases (for e.g: ACI) require that the public network be named as per the configuration setup in the SDN solution.

This commit is allowing the user to define the public network name as a variable from the Crowbar UI so that its possible to test other SDN backends requiring such flexibility.

[rsalevsky]

Seems like tempest is failing with this changes.