Add option to update releases while updating stemcell

[dlinsley]

In our automated system we always use the latest version of a stemcell and dependent compiled releases and have automation that attempts to keep the Kilnfile.lock current using update-stemcell and find-release-version / update-release.

On occasion:
when a new release and stemcell are created close together
and while trying to update the stemcell
the current release version in the Kilnfile.lock may not have a matching compiled release for the new stemcell.

Scenario example:

• compiled release exists for: release-v1-stemcell-v1
• release bumped: release-v2-stemcell-v1
• new stemcell available but build fails for: release-v2-stemcell-v2
• release bumped, build succeeds: release-v3-stemcell-v2

This leaves the following compiled releases available:

• release-v1-stemcell-v1
• release-v2-stemcell-v1
• release-v3-stemcell-v2

Since release-v2-stemcell-v2 is missing, kiln update-stemcell cannot update the Kilnfile.lock

The proposed change in this PR adds an optional --update-releases flag to update-stemcell to match the latest release for the specified stemcell given the version constraints in the Kilnfile and allow the prior scenario to update the Kilnfile.lock

Existing functionality without the --update-releases flag is retained.

[drich10]

Looks good to me, just one requested change and I'll go ahead and merge after 🙏