Allow customer set bond defaults for CL with ifupdown2

Allow customer set bond defaults for CL with ifupdown2

Ticket: CM-6723
Reviewed By: roopa
Testing Done: unit tested and testifupdown2 test suite

This patch installs bond interface defaults in

     /etc/network/ifupdown2/policy.d/bond_defaults.json

and allows users to modify this file.   Users can then leave out these
bond attributes in their configs to save typing and space.

It also changes the ifenslave and ifenslaveutil module to bond and
bondutil, respectively to be consistent with other modules
(and also because customers think of "bond" interfaces not
"ifenslave" interfaces.)

For example, the default file installed looks like the following:

{
    "README": "This file is user generated and modifiable.",
    "bond": {
        "defaults": {
                "bond-mode": "802.3ad",
                "bond-miimon": "100",
                "bond-use-carrier": "1",
                "bond-lacp-rate": "0",
                "bond-min-links": "1",
                "bond-xmic-hash-policy": "layer3+4"
         }
    }
}
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