openconfig-bgp-policy:asn/repeat-n needs clarification about default values

[mkh-arista]

Commit e020ce4 by @aashaikh added asn as the value used in conjunction with repeat-n to prepend the AS path with, but did not specify behaviour if asn is present but repeat-n is not.

Sane options are:

1. Refuse to accept asn without repeat-n
2. Prepend asn once
3. Do nothing

Option 1 could be implemented using when statement, but now it's probably too late for it may break existing configurations.

Option 2 essentially gives repeat-n two default values: 0 when asn is empty, and 1 otherwise: not great theoretically, though understandable practically. Whether this is a problem is subjective.

Option 3 seems safe unless 2 is already popular in the wild, which is hard for me to tell.

In any case, something needs to be done to prevent implementations divergence.