pool: make metadata directory location configurable

Motivation:
Many deployments prefer to keep pool's metadata on a different disk then
the data directory. As this is not possible out-of-box, typically this
is achieved with a help of sym-links

Modification:
introduce a property `pool.path.meta` to control location of medata
directory, that falls back to `pool.path`.

Result:
more flexible pool deployment

Acked-by: Marina Sahakyan
Target: master
Require-book: no
Require-notes: yes

modules/dcache/src/main/resources/org/dcache/pool/classic/pool.xml

@@ -113,7 +113,7 @@
                           <bean class="${pool.plugins.meta}" init-method="init">
                               <description>Store for pool meta data</description>
                               <constructor-arg ref="file-store"/>
-                              <constructor-arg value="#{ T(java.nio.file.FileSystems).getDefault().getPath('${pool.path}') }"/>
+                              <constructor-arg value="#{ T(java.nio.file.FileSystems).getDefault().getPath('${pool.path.meta}') }"/>
                               <constructor-arg value="${pool.name}"/>
                           </bean>
                       </constructor-arg>

skel/share/defaults/pool.properties

@@ -11,6 +11,13 @@ pool.name =
 # Path to pool directory
 pool.path =
 
+# Path to metadata directory base
+#
+# Note: this is the base directory where metadata implementation will create their
+# own subdirectories, like ${pool.path}/meta or ${pool.path}/control
+#
+pool.path.meta = ${pool.path}
+
 #  ---- Mover queues to create on a pool
 #
 #   Mover queues schedule and execute transfers on a pool. Each mover