Add --docker-registry and --image-tag flags to generator

[worxli]

This allows to run the dockerized topology without building docker images locally.
E.g. ./scion.sh topology -d --docker-registry scionproto --image-tag v0.x.x

This change is 

[kormat]

Reviewed 3 of 3 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @worxli)

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python/topology/docker.py, line 357 at r1 (raw file):

        ]

    def _docker_image(self, image):

How about putting this in common.py, and change it to docker_image(args, image)?

[worxli]

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @kormat)

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python/topology/docker.py, line 357 at r1 (raw file):

Previously, kormat (Stephen Shirley) wrote…

How about putting this in common.py, and change it to docker_image(args, image)?

Done.

[kormat]

Reviewed 7 of 7 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved

[kormat]

Reviewed 5 of 5 files at r3.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @worxli)

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docker/perapp/publish, line 10 at r3 (raw file):

# Tag local apps and push to remote
for app in border dispatcher sig cert path sciond beacon_py cert_py path_py sciond_py; do

I would order this by service type.

[worxli]

Reviewable status: 12 of 13 files reviewed, 1 unresolved discussion (waiting on @kormat)

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docker/perapp/publish, line 10 at r3 (raw file):

Previously, kormat (Stephen Shirley) wrote…

I would order this by service type.

Done.

[kormat]

Reviewed 1 of 1 files at r4.
Reviewable status: complete! all files reviewed, all discussions resolved