OVS agent: fix a comment on CANARY_TABLE

CANARY_TABLE is for br-int, not for br-tun.  Grouping it with
constants for br-tun tables together is misleading.  Move it into
a separate section and comment appropriately.  No functional changes.

Change-Id: Ic1aed992776a87ed77665b712de7ec3bd9dab397
Merge "OVS agent: fix a comment on CANARY_TABLE"