NSX allows DFW rules to be added to a DFW section with query
param 'operation', which can be used to create the said rule
at top or bottom within the DFW section. This patch
adds 'operation' as an argument for the methods 'add_rule'
and 'add_rules'.
Change-Id: I82dd206967543b3382dd20d7bffb140fce9cb59d