puppet: install git-client plugin explicitely in version 1.1.2

git-client plugin version 1.3.0 depends on ssh-agent which depends on
credentials plugin version 1.6 or newer. Otherwise it's failing with:

| SEVERE: Failed Loading plugin git-client
| java.io.IOException: Dependency ssh-agent (1.3) doesn't exist

Jenkins LTS 1.509.3 provides Credentials plugin version 1.3.1.
Jenkins 1.534 provides Credentials plugin version 1.4.

Recent versions of the git-client plugin (at least 1.3.0) depend on the
ssh-agent plugin which itself requires version >=1.6 of the Credentials
plugin though, otherwise it's failing with:

| FATAL: com/cloudbees/plugins/credentials/common/StandardCredentials
| java.lang.NoClassDefFoundError: com/cloudbees/plugins/credentials/common/StandardCredentials
|    at org.jenkinsci.plugins.gitclient.Git$1.invoke(Git.java:62)
| [...]

Manually updating the credentials plugin in an automated fashion doesn't
seem to be possible (when placing
http://updates.jenkins-ci.org/latest/credentials.hpi inside the plugin
directory then on Jenkins restart the built-in credentials plugin seems
to always get restored to its old/built-in version).

So unless there's a way to update a built-in plugin besides using the
web interface with its plugin manager we've to install a git-client
plugin version that's known to work without further, unresolvable plugin
dependencies.

Reported upstream as https://issues.jenkins-ci.org/browse/JENKINS-19927