MINOR: spoe: add force-set-var option in spoe-agent configuration

MINOR: spoe: add force-set-var option in spoe-agent configuration

For security reasons, the spoe filter was only able to change values of
existing variables. In specific cases (ex : with LUA code), the name of
variables are unknown at the configuration parsing phase.
The force-set-var option can be enabled to register all variables.