BUG# 1258262 Added secret parameter to options for sensitive data

Change-Id: Ib523b4df2540fde6972c1dcb9c8485c52f022dc4
Make sure that certfile and keyfile are readable

If the certfile or keyfile is missing or Designate doesn't have
permission to read them, nsd4slave would throw a cryptic SSL exception
when it tried to connect to the NSD4 servers. To avoid this, make sure
that the certfile and keyfile are readable upon backend init and raise a
human-readable exception if they're not.

Change-Id: I837cd328cf31f568e5da4ad4edf8c4dd34ddf8e2
Merge "BUG# 1258262 Added secret parameter to options for sensitive data"