allow configuring https.protocols and https.cipherSuites on Nexus outbound HTTP client connections

In NEXUS-5526, a change was made to allow javax.net.* system properties to be set on the HTTP Client embedded within Nexus.

https://github.com/sonatype/nexus-oss/commit/a7eb959b98b769cdfef190348f61e94b385ed32a#diff-35540aaccce768f70470408e7d8a0470

We have since regressed back to not allowing this:

https://github.com/sonatype/nexus-oss/commit/6c6507c8daf1cf6110d5f1d63c4d28a25b62c563#diff-35540aaccce768f70470408e7d8a0470

And the attempt to backout changes did not restore use of the System connection factory method:

https://github.com/sonatype/nexus-oss/commit/0ab135701ad42b169d8283fb8bddb110a3bd3bf7#diff-35540aaccce768f70470408e7d8a0470

In Nexus 2.8.0-05 we started including httpclient 4.3.x into Nexus. httpclient 4.3.x system SSLSocketFactory started honouring https.protocols and https.ciphersuites - however in 2.8 our regression prevents setting protocols and ciphers on outbound connections from being an option for end users.

So two problems:

• it seems we have regressed honouring system properties that influence the outbound http connections
• we don't expose a way to set https protocols and ciphers preferences on Nexus outbound connections ( https.protocols / https.ciphersuites )

Expected

• make Nexus use the system socket factories provided by http client, so that standard javax.net system properties can influence outbound connections

http://hc.apache.org/httpcomponents-client-4.3.x/httpclient/xref/org/apache/http/conn/ssl/SSLConnectionSocketFactory.html#162