Support scm ref for config repo configuration

[sahlone]

Need to support the scm ref for the plugin config itself. I am not talking about plugable scm for the pipeline.
Want to do something like this

<scms>
<scm id="7e0072f2-dd30-4cdc-b618-336be0e5e14d" name="Portal-pull-request">
<pluginConfiguration id="xxx" version="1" />
<configuration>
<property>
<key>url</key>
<value>mygit_repo</value>
</property>
<property>
<key>username</key>
<value>blablabla</value>
</property>
<property>
<key>password</key>
<encryptedValue>gjhgjhghgjhgjhghjgj</encryptedValue>
</property>
</configuration>
</scm>
</scms>

In server config cruise-config.yaml

<config-repo pluginId="yaml.config.plugin" id="QA-Merchant-Simulator">
      <scm ref="7e0072f2-dd30-4cdc-b618-336be0e5e14d" />
</config-repo>

This will help me deduplication of the same git repo configs at two places, one in scm and another in config. repo.