packaging: setup: NFS config on update change

Do not allow configuring an NFS ISO domain on upgrade
by default.

Allow user to override the new behavior of not
re-configuring NFS while upgrading.
User may now override the default behavior by setting
otopi environment variable:
 OVESETUP_SYSTEM/nfsConfigEnabled

Replace-Change-Id: I10734f3debd5e3fb122707deb0cb8b354fbe2fa6
Change-Id: I1fb4a9eb4af5e811a1fa1a63b8c11126b01ae56c
Related-To: http://gerrit.ovirt.org/26043
Bug-Url: https://bugzilla.redhat.com/1073464
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>