allow config admin to print out blueprint cm namespace default-properties values

Blueprint xml can use cm namespace to configure property placeholders. It might config default-properties:

<cm:property-placeholder persistent-id="com.redhat.fuse.samples.mytest"  update-strategy="reload">
      <cm:default-properties>
          <cm:property name="timeout" value="1000"/>
          <cm:property name="mykey" value="myvalue"/>
      </cm:default-properties>

The properties configured in <cm:default-properties> block will not be stored in OSGi config admin service. It means that the properties "timeout" and "mykey" won't be printed out by command "config:list":

JBossFuse:karaf@root> config:list |grep timeout

The OSGi config admin service only stores properties that are configured through "etc/${PID}.cfg" file. For instance, if I create an .cfg file called "etc/com.redhat.fuse.samples.mytest.cfg" and have the same set of properties in it:

timeout=1000
mykey=myvalue

then the properties "timeout" and "mykey" will be stored in OSGi config admin service. Then the same command will return:

JBossFuse:karaf@root> config:list |grep timeout
JBossFuse:karaf@root> timeout=1000
JBossFuse:karaf@root> config:list |grep mykey
JBossFuse:karaf@root> mykey=myvalue

We might consider to allow config admin service to print out those default-properties in blueprint even though there is no external "etc/{PID}.cfg" file with the same set of default-properties in it.