bgpd: When not fully connected to zebra gracefully ignore the issue

bgpd: When not fully connected to zebra gracefully ignore the issue

When bgp is coming up and is reading a non-integrated config.
The bgp connection to zebra has not fully had a chance to start.
As such when a redistribute line is parsed the attempt is
made to install it but it was erroring out with a warning.
This caused the `redistribute XXX` line to create a error
message to the end user.

Since bgp calls zclient_send_reg_requests which re-registers
the redistribute call once the actual zebra connection is up
and once bgp comes alive this is ok.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>