Unhardcode proxy_set_header

This patch unhardcode proxy_set_header options from proxy template.
The default behaviour is keept. Header might be passed via the following
pillar of site definition:

  nginx:
    server:
      site:
        abc:
          type: nginx_proxy
          proxy_set_header:
            My-Header:
              enabled: true
              value: '"MyValue"'
              
Related-Prod: PROD-23522

Change-Id: I8108b7ca0deab6b0c79a764beec3ca90e39d3f17
2 files changed