Leave the source hostname out of captured images

The image kept /etc/hostname of the source host, so a diskless node ran
under that name until onboot set its own, and early journal and syslog
entries carried the wrong name. With an empty /etc/hostname systemd
keeps the node name the deployment initramfs already set. Install to
disk writes /etc/hostname itself.

imgutil/imgutil

@@ -240,6 +240,8 @@ def capture_fs(args):
         masker.mask('/etc/pki/tls/private/*')
         masker.mask('/root/.ssh/id_*')
         masker.mask('/etc/netplan/*.yaml')
+        # an empty hostname keeps the node name the deployment initramfs set
+        masker.mask('/etc/hostname', '/run/imgutil/captmp/empty')
         mksquashargs = ['mksquashfs', '/run/imgutil/capin', fname + '.sfs', '-comp', 'xz']
         excludes = get_capture_excludes(mdir)
         exfile = None