Added NAT_Outgoing parameter to Calico Node

[VincentS]

This patch adds an NAT_Outgoing Parameter for IPv4 / IPv6 to Calico Node to be configurable on startup. This PR was moved from calicoctl Repository projectcalico/calicoctl#1640

Todos

• Tests
• Documentation

Release note

Ability to enable / disable outgoing NAT on the default IP Pool using an environment variable.

[tmjd]

From the test error, it looks like you need to check if nat-outgoing is present in pool['spec'].
I think you could just switch the whole block here to something like:

if "nat-outgiong" in pool['spec']:
  assert pool['spec']['nat-outgoing'] is nat_outgoing, \
    "Wrong NAT default in pool %s, expected nat-outgoing to be %s" % (pool, nat_outgoing)
else:
  assert nat_outgoing is False, \
    "Wrong NAT default in pool %s, expecting nat-outgoing to be disabled" % pool

I don't think it is necessary to indicate v4 or v6.

[tmjd]

LGTM
@VincentS could you rebase onto master and squash your commits into one please.
@projectcalico/calico-maintainers PTAL

[VincentS]

Rebased and squashed the commits
Documentation update can be found at #775

[nelljerram]

@tmjd I took a very quick look, but think you need @robbrockbank 's eyes here.

[robbrockbank]

@VincentS LGTM.

I think it's just missing a minor docs update. Would you mind adding an entry to the following doc?:

https://github.com/projectcalico/calico/blob/master/master/reference/node/configuration.md

If you prefer we can merge this as is and we'll add it later - let me know.

[tmjd]

@robbrockbank The PR #775 exists for the documentation. That PR existed before calico_node was moved into this repo. Do you think that needs to be merged with this PR? I'm of the opinion that it doesn't matter.

[robbrockbank]

@tmjd TBH I didn't spot that PR, I'm happy to merge this and update the docs later.