add ssl_verify_client parameter

[rchicoli]

the ssl_verify_client variable was hardcoded to 'on', now it is possible to pass other values like "on | off | optional | optional_no_ca".

[3flex]

Thanks for the code @rchicoli!

Couple of things:

• Test at https://github.com/jfryman/puppet-nginx/blob/master/spec/defines/resource_vhost_spec.rb#L930 is now failing. See Travis results e.g. https://travis-ci.org/jfryman/puppet-nginx/jobs/122227324). This line needs to be updated since the spacing in the output of the file is changed, so the existing matcher isn't matching.
• Please try adding some tests for the new parameter. You could model after these tests: https://github.com/jfryman/puppet-nginx/blob/master/spec/defines/resource_vhost_spec.rb#L390-L401

Cheers!

[rchicoli]

Thx for pointing out the error @3flex

• that was pretty easy to fix
• the ssl_verify_client parameter has to belong to some contexts, because it will only be set if ssl_client_cert is given

Glad to contribute.