installation: no user prompt for warnings

* Does not prompt user anymore for installation warnings that can be
  solved later, such as ffmpeg related warnings.  Useful for fully
  automated unassisted installations.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
Merge branch 'maint-1.1'

* maint-1.1:
  installation: no user prompt for warnings