Fix ci_stage_1.groovy to take ROBOT_TAG_NAME as a string

This change will read the param ROBOT_TAG_NAME and add it as a string
param to be passed to downstream jobs, instead of a boolean param, as
it was wrongly doing.

Change-Id: I3b7042dc41cd9a270386c91fdbaa7987e23af655
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>