vnf_generic: convert sshmanager to class

vnf_generic: convert sshmanager to class

@contextmanager have an issue with respect to exceptions that makes
them not suitable for real usage.

@contextmanager uses yield to create a generator and then uses
generator.throw() to raise any exceptions.  Exceptions thrown from
generators loose their call stack due to the way generators work, so any
exception inside a context manager is harder to debug. For this reason
we don't use @contextmanager and instead always define a new class with
__enter__ and __exit__.

There is sample code that demonstrates the
issue with @contextmanager and generator.throw() here
https://gist.github.com/rbbratta/e28b6e64a4551522c3ac9815ca7f25f0

Change-Id: I5383c01f40a63e33680112f39b5bd9c858e328f1
Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>