NEXUS-5472: Enabled and extended WL Client IT

WL Client IT was disabled (until merged) but then forgotten.
It is enabled now, and extended a lot to cover all
methods offered by Whitelist subsystem of the Nexus Client.
NEXUS-5472: Extending ITs for blocked/out of service cases
NEXUS-5472: Removed redundant checks

When execution is here, these were already checked.
NEXUS-5472: Logging cleanup

For those who care... ;)
NEXUS-5472: Adding reasons for unpublishing (for groups)

Listing members that causes for host to not have WL
Nexus Client Core extended with "directives" and "exists".

Directives support for local-only, remote-only, as-expired
and group-only requests. Also added exists methods to simply
detect existence of content.
NEXUS-5472: Sharing HTTP client

Moving it a level upper to share it across Whitelist
ITs
NEXUS-5472: WL Publishing IT with groups.
NEXUS-5472: WL publishing with hosted repo IT
NEXUS-5472: WL publishing with proxies IT