Clarify documentation and add logging around add-headers and proxy-set-headers

[ryanmckaytx]

Is this a request for help? : No, more of an offer of help if desired

What keywords did you search in NGINX Ingress controller issues before filing this one? : add-headers, configmap

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Is this a BUG REPORT or FEATURE REQUEST? (choose one): FEATURE REQUEST

It isn't obvious from the docs that the the value for add-headers and proxy-set-headers is the name of the configmap, including namespace. Also, if the value provided is incorrect (e.g. misspelled or missing explicit namespace), there is no log message indicating such. I propose the following:

1. Describe the format of the value in the docs (docs/user-guide/configmap.md)
2. Add logging here https://github.com/kubernetes/ingress-nginx/blob/master/internal/ingress/controller/nginx.go#L574 and https://github.com/kubernetes/ingress-nginx/blob/master/internal/ingress/controller/nginx.go#L586 if the specified config map was not found.

If this sounds good, let me know, I'll send a PR. Thanks!

NGINX Ingress controller version: 0.9.0-beta.15

Kubernetes version (use kubectl version):
Client Version: version.Info{Major:"1", Minor:"8", GitVersion:"v1.8.1", GitCommit:"f38e43b221d08850172a9a4ea785a86a3ffa3b3a", GitTreeState:"clean", BuildDate:"2017-10-12T00:45:05Z", GoVersion:"go1.9.1", Compiler:"gc", Platform:"darwin/amd64"}
Server Version: version.Info{Major:"1", Minor:"7", GitVersion:"v1.7.4", GitCommit:"793658f2d7ca7f064d2bdf606519f9fe1229c381", GitTreeState:"clean", BuildDate:"2017-08-17T08:30:51Z", GoVersion:"go1.8.3", Compiler:"gc", Platform:"linux/amd64"}

What happened:
No headers were added when the value for add-headers omitted namespace

What you expected to happen:
Headers to be added

How to reproduce it (as minimally and precisely as possible):
omit the namespace when specifying value for add-headers

[aledbf]

@ryanmckaytx thank you for the report. Please send the PR or PRs to change this.
Thanks!

[aledbf]

Closing. Fixed in #1853