Fix error in generated nginx.conf for optional hsts-preload

[matjazp]

With PR #563 a trailing space is rendered in nginx.conf for optional HSTS preload option (false by default):

more_set_headers                        "Strict-Transport-Security: max-age=15724800; includeSubDomains; ";

Error

This brakes some clients, e.g. very popular HTTP2 Python hyper-h2 with:

h2.exceptions.ProtocolError: Received header value surrounded by whitespace b'max-age=15724800; includeSubDomains; '

Fix

This PR fixes the trailing space and moves it before the optional value as " preload".

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[k8s-reviewable]

This change is 

[coveralls]

Coverage decreased (-0.03%) to 47.043% when pulling d402e16 on matjazp:fix-hsts-preload into bf5610d on kubernetes:master.

[matjazp]

I signed it

[aledbf]

/lgtm

[aledbf]

@matjazp thanks!