[UnitTest] Add very basic tests for NFSSR.py, ISOSR.py and nfs.py

Signed-off-by: Robert Breker <robert.breker@citrix.com>
Reviewed-by: Mate Lakat <mate.lakat@citrix.com>
CA-135612: Add experimental support for NFSv4 to SM

This enables the specification of the NFS version (3,4) in the device-config of
ISOSRs(type=nfs) and NFSSRs.
E.g. device-config:nfsversion=4

Signed-off-by: Robert Breker <robert.breker@citrix.com>
[Reviewed, improved]
Signed-off-by: Mate Lakat <mate.lakat@citrix.com>
[UnitTest] CA-135612: Add tests for nfsversions to NFSSR, ISOSR and nfs

Signed-off-by: Robert Breker <robert.breker@citrix.com>
Reviewed-by: Mate Lakat <mate.lakat@citrix.com>
ToDo: Default to using NFS version 3 again

We were unintentionally using nfsversioni 4 on trunk.
This was because the command mount.nfs defaults to using nfsversion 4 instead
of version 3 in our environment on trunk, despite it's documentation claiming
otherwise.
This commit specifies the nfsversion additionally in the mount options, to make
sure that we default to nfsversion 3 again, and that the default setting can
be switched intentionally.

Signed-off-by: Robert Breker <robert.breker@citrix.com>
[UnitTests] ToDo: Updated unittests to default to using NFS version 3 again

Signed-off-by: Robert Breker <robert.breker@citrix.com>