Introduce 'immutable' of parameters
Introduce 'immutable' of parameters

'immutable' is supported in parameters section,
add some guidlines for new attribute.

Change-Id: I8b102ffd2969d88ce19570764e10482cc52e49af

openstack/heat,steveb/heat,cwolferh/heat-scratch,cwolferh/heat-scratch,steveb/heat,openstack/heat,noironetworks/heat,noironetworks/heat