[Public] Make not to modify gbs.conf when tizen version is upgraded

[Public] Make not to modify gbs.conf when tizen version is upgraded

Whenever the latest tizen version is released, gbs.conf has been modified
to add new repositories. It was painful routine.

With this changes, the gbs.conf doesn't need to be modified anymore and
all we have to do is just setting |DEFAULT_TIZEN_VERSION| to proper version
in common.sh like below.

> export DEFAULT_TIZEN_VERSION=5.5

Change-Id: I7c288d221d2d7eb3eba5cf4eb7894c24f6ffea6e
Signed-off-by: Youngsoo Choi <kenshin.choi@samsung.com>