loman: optionally set revision element
loman: optionally set revision element

since ElementTree doesn't know how to make an element with "None",
only create the revision attribute if it is not None.

BUG=chromium-os:33688
TEST=verified that a remote project with and without revisions works

Change-Id: I568a9e2821a1c089a0ba391f6df6c0e3b2dc61c9
Signed-off-by: Rhyland Klein <9c6f026f68ffecc44f215af186ef513cd72b6ce4@nvidia.com>
Reviewed-on: https://gerrit.chromium.org/gerrit/32393
Commit-Ready: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Reviewed-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Tested-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>

chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,chadversary/chromiumos.chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,zhang0137/chromite,coreos/chromite,zhang0137/chromite