fuel: Normalize wrappers.ssh-agent-credentials.users

fuel: Normalize wrappers.ssh-agent-credentials.users

This changes
        - ssh-agent-credentials:
            users: '{ssh-credentials}'

to
        - ssh-agent-credentials:
            users:
                - '{ssh-credentials}'

Then, the size of the generated XML can be slightly smaller, such
as:

Old XML:

    <com.cloudbees.jenkins.plugins.sshagent.SSHAgentBuildWrapper>
      <credentialIds>
        <string>d</string>
        <string>4</string>
        <string>2</string>
        ...
        <string>b</string>
        <string>2</string>
        <string>e</string>
      </credentialIds>
    </com.cloudbees.jenkins.plugins.sshagent.SSHAgentBuildWrapper>

New XML:

    <com.cloudbees.jenkins.plugins.sshagent.SSHAgentBuildWrapper>
      <user>d42411ac011ad6f3dd2e1fa34eaa5d87f910eb2e</user>
    </com.cloudbees.jenkins.plugins.sshagent.SSHAgentBuildWrapper>

Change-Id: I8215a33784c63757a5b74f30a5aa771fc19c5207
Signed-off-by: Zhijiang Hu <hu.zhijiang@zte.com.cn>