addons: address: reset mtu to default for physical interfaces which are

addons: address: reset mtu to default for physical interfaces which are
not bond slaves

Ticket: CM-9449
Reviewed By: julien
Testing Done: Tested resetting mtu on physical interfaces to default

default mtu is already present in address.json policy file. This patch
continues to take a cautious and conservative approach to
reset mtu of only physical interfaces if different from default mtu
and mtu is not given by the user. bond slaves get their mtu from
the bond when bond has a configured mtu. so this patch only
resets mtu on physical interfaces which are not bond slaves.

{noformat}
$cat /var/lib/ifupdown2/policy.d/address.json
{
    "address": {
	"defaults": { "mtu": "1500" }
    }
}
{noformat}

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>