Remove omitempty tag from Machine.Spec.Taints

[dlipovetsky]

What this PR does / why we need it:
Allows the user to specify an empty set of taints (as opposed to specifying nothing at all).

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #707

Special notes for your reviewer:
This is a breaking API change, please hold until after v1alpha1.

Remove `omitempty` tag from `Machine.Spec.Taints` field to allow users to specify an empty set of taints.

[k8s-ci-robot]

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[dlipovetsky]

Breaking API change. Should not be merged until v1alpha1 release.
/hold

[detiber]

/milestone Next

[k8s-ci-robot]

@dlipovetsky: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
pull-cluster-api-test	657b2eb	link	/test pull-cluster-api-test

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[detiber]

Closing since we probably shouldn't modify the v1alpha1 types at this point. We should incorporate this into the v1alpha2 work, though.
/close

[k8s-ci-robot]

@detiber: Closed this PR.

In response to this:

Closing since we probably shouldn't modify the v1alpha1 types at this point. We should incorporate this into the v1alpha2 work, though.
/close

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