Clear timeserver on boot time

Clear timeserver on boot time

Currently, timeserver is cleared after a period of time after setting.
However, in this method, there is a problem that timeserver cannot be
retrieved if the interface has no internet connection is connected
first, and then the interface has internet connection is connected
just before the timeserver is cleared.
Therefore, it has been modified so that timeserver is not cleared
until rebooting.

Change-Id: I2f5ca9f6d01bf7be2668338f3a159a401a0d46b5
Signed-off-by: Jaehyun Kim <jeik01.kim@samsung.com>