[XrdSsi] Updates CTA Frontend config to use SSI+OFS plugins

xroot_plugins/xrootd-cta.cfg

-# Load the plugin XrdCtaOfs
-xrootd.fslib libXrdCtaOfs.so
+# Load the SSI + OFS plugins
+xrootd.fslib libXrdSsi.so libXrdCtaOfs.so
+
+# Specify which paths are handled by the OFS plugin
+#
+# Y3Rh is the Base64 encoding of cta, so this string appears at the beginning
+# of all cta admin commands
+ssi.fspath /Y3Rh
+
+# Load the SSI module
+ssi.svclib libXrdSsiCta.so
 
 # Use the security module
 xrootd.seclib libXrdSec.so
 
 # Protocol specification
+#
 # The xroot server process needs to be able to read the keytab file
 sec.protocol krb5 /etc/krb5.keytab.cta host/<host>@CERN.CH
 sec.protocol sss -s /var/spool/xrootd/.xrd/sss.keytab -c /tmp/sss.keytab
@@ -13,8 +23,15 @@ sec.protocol sss -s /var/spool/xrootd/.xrd/sss.keytab -c /tmp/sss.keytab
 sec.protbind * only sss
 sec.protbind tpsrv* only krb5
 
-# Allow copying from absolute paths
-all.export /
+# Export the SSI resource
+all.export /ctafrontend nolock r/w
+
+# Export the OFS resource, specified in ssi.fspath above.
+#
+# There can be more than one of these; anything that starts with the prefix
+# will go to the the underlying file system. Note that this prefix cannot be
+# a substring of '/ctafrontend', so it is not possible to simply export '/'.
+all.export /Y3Rh
 
 # Turn off asynchronous i/o
 xrootd.async off