Set YCM option YCM_DISABLE_SYSTEM_PACKAGES to OFF

During the past years, we experienced quite a lot of times that for some reason on a setup 
the superbuild was finding its own installed packages, or finding the one of a similar superbuild 
installed on another setup. As recovering from this kind of errors is tricky for users, and as 
most of robotology-superbuild users do not need to find  the packages installed by the superbuild in 
their system, the YCM_DISABLE_SYSTEM_PACKAGES option was introduced in YCM 
( see robotology/ycm-cmake-modules#332 )  to disable the use of 
any system installed package if the same package can be installed by the superbuild (equivalent of setting 
all the USE_SYSTEM_<project> options to OFF). 

This option is set by default to `OFF` on YCM for backward compatibility, but to avoid all the problems in the superbuild
we set it to `ON` by default. If an expert user want to be able to find system-installed packages (for example because he has a 
system-installed YARP) he just need to set it at `ON`. 

The option is still not supported in the stable released version of YCM, so I will not update the README for now, but 
it will be already effective if a users specifies uses `Unstable` `ROBOTOLOGY_PROJECT_TAGS`, so for example it  will be 
already useful  for the iCubGenova01 setup @randaz81 @xEnVrE .

Related issues: 
* #121
* #174

CMakeLists.txt

@@ -26,7 +26,8 @@ include(FeatureSummary)
 
 set(YCM_USE_CMAKE_PROPOSED TRUE CACHE BOOL "Use files including unmerged cmake patches")
 
-# Compilation options
+# YCM options 
+option(YCM_DISABLE_SYSTEM_PACKAGES "Disable use of all the system installed packages" ON)
 
 # Dependencies options 
 ## Matlab related options