[enh] make backup and restore scripts multiinstance too

scripts/backup

@@ -3,7 +3,7 @@
 # causes the shell to exit if any subcommand or pipeline returns a non-zero status
 set -e
 
-app=ynhexample
+app=$YNH_APP_INSTANCE_NAME
 
 # The parameter $1 is the backup directory location
 # which will be compressed afterward

scripts/restore

@@ -3,7 +3,7 @@
 # causes the shell to exit if any subcommand or pipeline returns a non-zero status
 set -e
 
-app=ynhexample
+app=$YNH_APP_INSTANCE_NAME
 
 # The parameter $1 is the uncompressed restore directory location
 backup_dir=$1/apps/$app