psdk-test fix for session close and proxy issue.

psdk-test case for etc was failing as session was not getting closed.
psdk-test case for sysroot was failing as proxy setting step was not done properly.

Fix: paramiko library will be used, which have option of closing the session
after running particular command.
psdk-test for sysroot steps has been modified for setting up proxy and then running the
ade sysroot commands.

Apertis: T7237, T7238

Signed-off-by: Tarun Baghmar <tarun.baghmar@in.bosch.com>