[refactor] Separate CM and image in s2i

[gaocegege]

Ref #https://github.com/caicloud/ciao/pull/43#discussion_r221487888

Is it still correct to say s2i now that with configmap, we don't really build an image.

I can live with this if we think the image in s2i means the pre-defined image + code, but considering what the s2i project doest (actually building an image), this can be a little confusing.

[gaocegege]

/kind enhancement

[caicloud-bot]

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/lifecycle stale

[gaocegege]

/priority p1