isisd: 'tiebreaker' command line funtionality is inconsistent with its implementation

isisd: 'tiebreaker' command line funtionality is inconsistent with its implementation

The command fast-reroute lfa tiebreaker [downstream | lowest-backup-metric | node-protecting] index (1-255) [level-1 | level-2] will overwrite configurations with the same index but different types. This is because the index is set as the key in frr-isisd.yang. However, the lfa_tiebreaker_cmp function uses a tuple (index, type) as the key. Therefore, the yang file should be modified to stay in sync with the business logic.

Test Scenario:
On RouterA, first configure fast-reroute lfa tiebreaker downstream index 100 level-1, then configure fast-reroute lfa tiebreaker lowest-backup-metric index 100 level-1, and check the configuration:

!
router isis 10
 fast-reroute lfa tiebreaker lowest-backup-metric index 100 level-1
exit
!

Signed-off-by: baozhen-H3C <bao.zhen@h3c.com>