`ota_updater` os_version checks needs major-minor-patch

Right now the cloud only sends over major.minor for most things, but could possibly send all 3. However, when converting with `SemanticVersion.FromString`, that method requires the patch or it errors out.

One of two things needs to change:
1) `semver` should imply a 0 for patch if not provided
2) `ota_updater` should imply a 0 for patch if not provided, so that calling `FromString` works.

@timburke is there anything preventing us from altering the semver behavior, or do you have an opinion on what to change? Technically we could also update the Cloud's expectation for os_version and app_version to require the full major.minor.patch. Right now I don't believe the cloud has any enforcement mechanism for sanitizing the update requirements, but that's just what I derived from the project docs.