Changed 'xrootd.fslib libXrdEosMgm.so' to 'xrootd.fslib -2 libXrdEosMgm.so' so...

Changed 'xrootd.fslib libXrdEosMgm.so' to 'xrootd.fslib -2 libXrdEosMgm.so' so that XrdSfsGetFileSystem2() is called which in turn tells XRootD that an alternative Prepare plugin is present.

continuousintegration/docker/ctafrontend/cc7/config/ctaeos/etc/xrd.cf.mgm

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-xrootd.fslib libXrdEosMgm.so
+# A prepare request with the Prep_EVICT flag can only call
+# XrdMgmOfs::prepare() if XRootD believes an alternative
+# Prepare plugin is present.  "xrootd.fslib -2" invokes
+# XrdSfsGetFileSystem2() which tells XRootD that such
+# a plugin is present.
+xrootd.fslib -2 libXrdEosMgm.so
 xrootd.seclib libXrdSec.so
 xrootd.async off nosf
 xrootd.chksum adler32