Extend tag feature of release jobs

Extend tag feature of release jobs

Extend release-yaml schema files to allow git_tag entry.
Extend lf-build-with-parameters-maven-release macro with GIT_TAG.
Extend release-job.sh:
  * detect and use optional git_tag string.
  * detect and reject an existing lightweight tag that blocks
    push of a gpg-signed tag.
  * change GERRIT_HOST to GERRIT_URL in method for obtaining LF
    umbrella project to allow testing in the sandbox.
  * rename function from tag to tag-gerrit-repo.
  * simplify tests for presence of Jenkins parameter values
  * show more details about steps as INFO statements
Include schema file contents into documentation; remove the copies.

Change-Id: Iee2167b059c7f0ecf86ab6eb8dead36ec96be1b5
Issue-ID: RELENG-2568, RELENG-2586
Signed-off-by: Lott, Christopher (cl778h) <cl778h@att.com>