bgpd: fix several issues in sourcing AIGP attribute

bgpd: fix several issues in sourcing AIGP attribute

Fix several issues in sourcing AIGP attribute:

1) AIGP should not be set as default for a redistributed route or a
   static network. It should be set by config instead.

2) AIGP sourced by "set aigp-metric igp-metric" in a route-map does
   not set the correct value for a redistributed route.

3) When redistribute a connected route like loopback, the AGIP (with
   value 0) is sourced by "set aigp-metric igp-metric", but the
   attribute is not propagated as the attribute flag is not set.

Signed-off-by: Enke Chen <enchen@paloaltonetworks.com>
(cherry picked from commit f65356d8bb9a43b1725fafdbd30aba0de9d214fa)