timedrift.cfg: add full_load variant for with_load

Add full_load variant for with_load, at the same time change the
structure of ntp and date variants.

change history:
V3: use default_load as variant name
V2: change the variant name 'default' to 'shared_ntp_date'
V1: add full_load variant for with_load, change the variants structure

Signed-off-by: Xiangmin Han <xhan@redhat.com>