Move mainSnippet before events to fix load_module issue.

[Globegitter]

What this PR does / why we need it:
I was trying to activate the dynamic geoip2 module added in 0.16 as follows:

main-snippet: |
    load_module /etc/nginx/modules/ngx_http_geoip2_module.so;

but then getting the error:

Error: exit status 1
nginx-ingress-controller 2018/10/02 13:24:16 [emerg] 21791#21791: "load_module" directive is specified too late in /tmp/nginx-cfg246481140:21
nginx-ingress-controller nginx: [emerg] "load_module" directive is specified too late in /tmp/nginx-cfg246481140:21
nginx-ingress-controller nginx: configuration file /tmp/nginx-cfg246481140 test failed

And turns out the load_module commands need to be listed before the event block.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

[aledbf]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, Globegitter

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[aledbf]

@Globegitter thanks!