node-proxy: refactor entrypoint

node-proxy: refactor entrypoint

This commit introduces a major refactor of the main
entrypoint.

- subclass threading.Thread:
  - Introduce a new class `BaseThread()` that is a
    `threading.Thread()` abstraction class in order
    to monitor the different threads.
  - `BaseSystem()` inherits from `BaseThread()`.
  - Handle `SIGTERM` signal in order to gracefully shutdown
    node-proxy (make threads exit gracefully, log out from RedFish API, etc.)

Additionally, this:
  - drops the class `Logger()` from util.py which
was not adding value. It is now replaced with a simple `get_logger()`
function.
  - changes the node-proxy API port from 8080 to 9456
    (8080 being widely used for frontend apps...)
  - changes the container entrypoint in order to use the
    `ceph-node-proxy` binary from the packaging

Signed-off-by: Guillaume Abrioux <gabrioux@ibm.com>