Use parameter_defaults in env files

Use parameter_defaults in env files

Switch to using parameter_defaults in environment files instead of a
parameters section. Using a parameters section to set top level
parameters breaks Tuskar based deployments because Tuskar prefixes the
name of the top level parameters with a role name and version, thus
changing the name of the parameter. When the environment file is then
used to set a top level parameter, Heat fails with an error during
template validation:

ERROR: The Parameter (NeutronExternalNetworkBridge) was not defined in template

Change-Id: I605651a8ebdbd0b2baf7bcea198c2988efb55f31