allow simple use of HTTP proxy for outbound S3 traffic

Background

• Currently, S3 blob store requests don't use the global proxy settings.
• Users can override properties to use a proxy, but this isn't documented.

Acceptance

• Allow admins setting up an S3 blob store to specify
  • no proxy
  • use global proxy settings (the default?)
  • proxy settings specific to this S3 blob store

Workaround

Configure the proxy server via bin/nexus.vmoptions using standard java system properties:

-Dhttp.proxyHost=proxy.x.y.z
-Dhttp.proxyPort=8080
-Dhttps.proxyHost=proxy.x.y.z
-Dhttps.proxyPort=8080
-Dhttp.nonProxyHosts="169.254.169.254|localhost|*.x.y.z"