NEXUS-6164: Expose SSL cert check lax options

ConnectionInfo now carries flags about relaxing the
X509 hostname check and SSL certificate check,
thus it's possible to make Nexus client accept
self signed certificates and certificates with
different X509 hostname that the name of the
host it connects to.

Also, client used default connection manager
(usable in single thread environment), but this
time it's set explicitly and made to honor
JSSE system properties, allow greater customization.

Issue
https://issues.sonatype.org/browse/NEXUS-6164

CI
TBD