Better handling of injected MetadataProvider

SHIBBOLETH_SP_METADATA_PROVIDER_XML may still be injected
using the _FILE approach but it is handled uniquely now
to prevent an error message from the bash declare
command caused by the contents of the file usually containing
special characters. Fundamentally this is caused by the
left and right-hand sides of

declare "${config_var}"=`cat $file_name`

being evaluated before the actual 'declare' command is evaluated.

comanage-registry-shibboleth-sp/docker-comanage-shibboleth-sp-entrypoint

@@ -28,13 +28,16 @@ fi
 
 # Configuration details that may be injected through environment
 # variables or the contents of files.
+#
+# SHIBBOLETH_SP_METADATA_PROVIDER_XML may also be injected in the
+# same way but because of the presence of special characters in the
+# XML it is handled differently.
 
 injectable_config_vars=( 
     SHIBBOLETH_SP_ENTITY_ID
     SHIBBOLETH_SP_CERT
     SHIBBOLETH_SP_PRIVKEY
     SHIBBOLETH_SP_SAMLDS_URL
-    SHIBBOLETH_SP_METADATA_PROVIDER_XML
 )
 
 # If the file associated with a configuration variable is present then