RHSM: when using proxy, test its connectivity first

RHSM: when using proxy, test its connectivity first

When using RHSM with a proxy, we want to make sure the proxy can be
reached. This patch verify that a tcp socket can be open from the client
to the proxy.

This patch also does a bit of refactoring:
- --retry-delay 10 --max-time 30 is now used in a parameter everytime we
  use curl.
- proxy options are now used everytime curl is used, even for detecting
  which version of Satellite is running, now we use proxy options.

Co-Authored-By: Vincent S. Cojot <vincent@cojot.name>
Change-Id: I4dcac1528c10f698338383445e27c8a613f9bcd9
Closes-Bug: #1724970
(cherry picked from commit f4e46f4b3ddac3f536a3a1955c91447e8b26ffca)