Add security to management of processes through supervisord
As part of this commit we do following:

a) Allow supervisorctl to connect to supervisord only through unix
domain sockets. We also make the permissions of supervisord unix
domain sockets restrictive so that only root can invoke supervisorctl
command.
b) Make changes in nodemgr so that we do not depend on Juniper
proprietary extensions to supervisord.

Change-Id: I991b81e78c94ad9224ac34efaf4dbf99a4e517b6
Closes-PR: 1383939
Closes-PR: 1383503