[NEXUS-5742] Create an auth/proxy auth preconfigured http client and avoid context based credentials provider

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
Merge pull request #28 from sonatype/nexus-5742-pr-via-auth-proxy

REVIEW: [NEXUS-5742] Create an auth/proxy auth preconfigured http client and avoid context based credentials provider