Use release credential in pypi release merge

Add macro with lf-infra-wrappers block to set jenkins-ssh-credential
parameter to the value in parameter jenkins-ssh-release-credential,
which makes the PyPI release merge templates parallel to the release-job
merge template.  Both need privileges to push a tag from Jenkins.
Document the revised configuration parameter. Silence yamllint issues.

Change-Id: I534c659ec4c5165184bc1de8bb7021dab116deab
Issue-Id: RELENG-2317
Signed-off-by: Lott, Christopher (cl778h) <cl778h@att.com>