Added simple way to compile all Coman models at the same time.

When compiling models using create_urdf_srdf_sdf.sh --all they all get copied
inside the coman_gazebo/database folder. All possible configurations are automatically
parsed from the urdf/conf folder.
The file urdf/conf/coman.urdf.xacro is the default file that is copied in the coman_urdf/urdf
folder as coman_config.urdf.xacro when compiling.
The coman_gazebo/database folder is a gazebo models folder, so by setting GAZEBO_MODEL_PATH
to point that folder, they will be automatically shown in gazebo.
The default installation by CMakeLists now does not copy files in ~/.gazebo anymore,
but the create_urdf_srdf_sdf.sh still does, for compatility reasons.
The files will not get copied inside ~/.gazebo/models/coman_urdf but inside
~/.gazebo/models/coman
Fixed incorrectly switched logic flag GAZEBO_COMAN_USES_SOFTHANDS_DRC_sw1_14