kubeadm: add mandatory configuration to "phase preflight"

[neolit123]

What this PR does / why we need it:
Add the:

• mandatory flag "--config" to the preflight phase
  and parse the specified config file for either "master" or "node".
• flag "--ignore-preflight-errors" to the preflight phase to
  allow skipping errors.
• the function AddIgnorePreflightErrorsFlag()
  to "options/generic.go", because the flag is used in multiple commands.

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 kubernetes/kubeadm#924

Special notes for your reviewer:
this is following:
kubernetes/kubeadm#924 (comment)

Release note:

kubeadm: add mandatory "--config" flag to "kubeadm alpha phase preflight"

@kubernetes/sig-cluster-lifecycle-pr-reviews
/assign @fabriziopandini
/milestone 1.12
/kind bug

[k8s-ci-robot]

@neolit123: You must be a member of the kubernetes/kubernetes-milestone-maintainers github team to set the milestone.

In response to this:

What this PR does / why we need it:
Add the:

• mandatory flag "--config" to the preflight phase
  and parse the specified config file for either "master" or "node".
• flag "--ignore-preflight-errors" to the preflight phase to
  allow skipping errors.
• the function AddIgnorePreflightErrorsFlag()
  to "options/generic.go", because the flag is used in multiple commands.

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 kubernetes/kubeadm#924

Special notes for your reviewer:
this is following:
kubernetes/kubeadm#924 (comment)

Release note:

kubeadm: add mandatory "--config" flag to "kubeadm alpha phase preflight"

@kubernetes/sig-cluster-lifecycle-pr-reviews
/assign @fabriziopandini
/milestone 1.12
/kind bug

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[fabriziopandini]

@neolit123 many thanks for this quick fix
LGTM

/assign @timothysc
for adding required labels/milestone

[timothysc]

/approve

It generally lgtm but I'd like at @fabriziopandini to dbl-check for final lgtm.

[fabriziopandini]

@neolit123 not blocking, but could you kindly add fmt.Println("[preflight] running pre-flight checks") + eventually a pre-flight checks passed after the error check?
This will provide a better UX (now the user doesn't gets any message in case all the checks are passing).
same for nodes

[neolit123]

@fabriziopandini i have added the [preflight] ... output to master and node
PTAL.

[fabriziopandini]

@neolit123 thanks!
/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: fabriziopandini, neolit123, timothysc

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• cmd/kubeadm/OWNERS [fabriziopandini,timothysc]

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[fabriziopandini]

/retest