standardize ssh auth

standardize ssh auth

we need to be following defautl paramiko rules,
first use pkey, then key_filenames (autodetecting ~/.ssh/ keys),
then password

We have too much boilerplate redudant code everywhere, we need
to standardize on a factory function that takes a node dict.

Using Python3 ChainMap we can layer overrides and defaults.

VNF descriptors have to default key_filename, password to Python None.
The only way to do this is to omit key values if the variable is not
defined, this way the dict will not have the value and it will
default to Python None

Add python2 chainmap backport

Updated unittest mocking to use ssh.SSH.from_node

Change-Id: I80b0cb606e593b33e317c9e5e8ed0b74da591514
Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>
(cherry picked from commit 99abbb424007da2e01762f3c040a39c0157cbe1f)