Get all pre-flight files into separate folder

[coolbrg]

This is a follow up issue of refactor task from this comment #1997 (comment).

We have many pre-flight files now inside cmd/minishift/cmd folder.
We can create one folder preflight and store all the existing (or future) files in it to have better separation.

Note: This will also help in readability when we will use in start.go as preflight.preflightChecksBeforeStartingHost.
May be now function name should be renamed to checksBeforeStartingHost as preflight word is not needed once we have its package.

[gbraad]

preflight.preflightChecksBeforeStartingHost should be something like preStartingHost and we need a postStartingHost generic call... as we can also implement this also in Minikube.

[stale]

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