Bypass engine via annotation

[ukclivecox]

• Allow direct model API calls for single model scenarios
• Activated by an annotation in the predictor seldon.io/no-engine: "true"
• Updates python wrapper to allow direct external API calls

This PR also:

• Adds some initial BDD tests to operator (Need to include istio CRD)

[ukclivecox]

/assign @ryandawsonuk

[axsaucedo]

/cc @ryandawsonuk

@cliveseldon I actually found it quite confusing but it seems that with Prow the command / assign only subscribes them, whilst / cc (and / uncc) is what adds/removes people as reviewers

[axsaucedo]

Wow, the change in the Python client is amazingly simple. Really cool!

[ryandawsonuk]

/approve

[seldondev]

[APPROVALNOTIFIER] This PR is APPROVED

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