lib: plist should not CMD_WARNING when command has already been entered

lib: plist should not CMD_WARNING when command has already been entered

When you startup zebra and apply a prefix list command, and then at a
later point in time start up additional daemons and then attempt to
apply the integrated-config, the prefix-list command will stop
processing after zebra.

This is because when zebra attempted to process the command, it would
notice that it already had that cli handled and print a vty warning and
then it would return CMD_WARNING. This caused the loop over all the
commands to stop and as such the command would not be sent to all the
individual daemons.

This behavior is exactly the same as it is in the 2.5.X( hell it's
upstream behavior to! ). Modify the plist command to return CMD_SUCCESS
in this case.

Ticket: CM-10248
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by: Don Slice <dslice@cumulusnetworks.com>