Added nginx config property 'ssl_client_certificate' to be changed

Description

Added the ssl_client_certificate nginx property to gitlab omnibus config file. You should now be able to do

nginx['ssl_client_certificate'] = '/path/to/ca.crt'

to add the root client certificate to the nginx configuration.

Why?

When trying to add a certificate for a not yet trusted vendor, you get back this error when trying to fetch over https:

fatal: unable to access '<snipped website>': server certificate verification failed. CAfile: /etc/ssl/certs/ca-certificates.crt CRLfile: none

It also adds more configuration possibilities to omnibus.