[NEXUS-5214] Add support for setting https proxy via nexus client

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
[NEXUS-5214] Add support for setting web proxy on repository

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)

The ITs are using the new IT support in nexus and do not depend anymore on external resources.
Will drop the ITs in old format that were depending on external resources

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