Disable LVM devices file when running image installs

The new blivet flag allows us to control whether the LVM devices
file will be populated with PVs or not and we don't want to write
the file when running an image installation.

Resolves: rhbz#2247872

pyanaconda/modules/storage/initialization.py

@@ -41,8 +41,11 @@ def enable_installer_mode():
 
     # We don't want image installs writing backups of the *image* metadata
     # into the *host's* /etc/lvm. This can get real messy on build systems.
+    # Same applies to system.devices file that shouldn't be populated with
+    # PV names from the system that creates the image
     if conf.target.is_image:
         blivet_flags.lvm_metadata_backup = False
+        blivet_flags.lvm_devices_file = False
 
     # Set the flags.
     blivet_flags.auto_dev_updates = True