Fix invalid port in upstream

[gianrubio]

If you don't have the ingress port mapped same as the service, you got the error "invalid port in upstream" on nginx. To fix this, we should check if the targetPort is valid (>0).
Fix #53

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[k8s-reviewable]

This change is 

[gianrubio]

I signed it!

[coveralls]

Coverage decreased (-0.02%) to 39.753% when pulling f0738f7 on gianrubio:branch/invalid-port into 0af8ccc on kubernetes:master.

[coveralls]

Coverage decreased (-0.02%) to 39.753% when pulling f0738f7 on gianrubio:branch/invalid-port into 0af8ccc on kubernetes:master.

[coveralls]

Coverage decreased (-0.02%) to 39.753% when pulling f0738f7 on gianrubio:branch/invalid-port into 0af8ccc on kubernetes:master.

[coveralls]

Coverage decreased (-0.02%) to 39.753% when pulling f0738f7 on gianrubio:branch/invalid-port into 0af8ccc on kubernetes:master.