[NEXUS-5214] Default to HTTP proxy if no HTTPS proxy is defined in same context

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
[NEXUS-5214] Add ITs related to which proxy is used (global or repository level defined ones), depending on url scheme (http / https)

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>