Allow the build process to receive a version qualifier at compile time

[ph]

Allow passing a build time qualifier so that a build artifact can be promoted to GA without requiring a rebuild. Our build process accepts the SNAPSHOT environment variable to add the snapshot suffix, and we need to add a VERSION_QUALIFIER options that will allow the release manager to specify the qualifier to be used.

The generated package will use the specified qualifier and the beats will also report their version using the qualifier used at built time.

[ph]

Linked to APM elastic/apm-server#1348

[graphaelli]

I'd like to request some feedback on elastic/apm-server#1348 (comment), all of the changes made were in vendored beats code.

[tsg]

@graphaelli I saw your comment only after pushing this PR: #8271. That is the result of Andrew walking me through the changes that are needed.

Your PR seems to change the code version (e.g. changes in GetDefaultVersion), we've strictly altered the final package names, nothing in the contents of them.

[graphaelli]

Right, I've tried to handle both the code version and package name. The tricky part was allowing each branch to set a default qualifier without requiring the build to provide one, or mage to know about the defaults - that is in 6.4.1 there is no qualifier by default, in 7.0.0 it's alpha1 unless otherwise specified.

[jasontedor]

We will not need default qualifiers as in the future all of our 7.0.0 snapshot builds will be unqualified. Qualifiers will only be used when preparing pre-releases.