samples/net/mqtt: Simplify MQTT publisher

Remove the global bluemix flag and use a #define to set the
MQTT publisher topic and its parameters at compile time.

This change will save a few bytes and speed up computations.

Change-Id: I27bfc6b38c73d32c6105f1d506e147e9a5583097
Signed-off-by: Flavio Santes <flavio.santes@intel.com>