Document passing of ssl_client_cert to backend

[tobilarscheid]

this fixes #700

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[k8s-reviewable]

This change is 

[tobilarscheid]

CLA is signed

[coveralls]

Coverage increased (+0.05%) to 47.139% when pulling 8d29610 on tobilarscheid:patch-1 into 317f222 on kubernetes:master.

[rikatz]

Actually the header in template is ssl-client-cert and not ssl_client_cert.

[rikatz]

@aledbf Just have to wait to correct the variable from ssl_client_cert to ssl-client-cert, and then LGTM :)

[coveralls]

Coverage decreased (-0.1%) to 46.983% when pulling 9c56c72 on tobilarscheid:patch-1 into 317f222 on kubernetes:master.

[rikatz]

LGTM

[aledbf]

/lgtm

[aledbf]

@tobilarscheid thanks!