allow parameters override from executor script

the proposed fix will allow to override the parameters in the sensu-client script for example 

$app="client"
export CONFIG_FILE=/etc/sensu/$app/config.json
export CONFIG_DIR=/etc/sensu/$app/conf.d
export EXTENSION_DIR=/etc/sensu/$app/extensions
export PLUGINS_DIR=/etc/sensu/$app/plugins
export HANDLERS_DIR=/etc/sensu/$app/handlers
export PID_DIR=/var/run/sensu/$app