[leapp] Unable mount all partitions during the upgrade to RHEL 8

Issue

During the reboot step, the python traceback below is printed out:

Jan 26 10:12:10 localhost upgrade[663]: ==> Processing phase `InitRamStart`
Jan 26 10:12:10 localhost upgrade[663]: ====> * remove_upgrade_boot_entry
Jan 26 10:12:10 localhost upgrade[663]:         Remove boot entry for Leapp provided initramfs.
Jan 26 10:12:11 localhost upgrade[663]: Process Process-158:
Jan 26 10:12:11 localhost upgrade[663]: Traceback (most recent call last):
Jan 26 10:12:11 localhost upgrade[663]:   File "/usr/lib64/python2.7/multiprocessing/process.py", line 258, in _bootstrap
Jan 26 10:12:11 localhost upgrade[663]:     self.run()
Jan 26 10:12:11 localhost upgrade[663]:   File "/usr/lib64/python2.7/multiprocessing/process.py", line 114, in run
Jan 26 10:12:11 localhost upgrade[663]:     self._target(*self._args, **self._kwargs)
Jan 26 10:12:11 localhost upgrade[663]:   File "/usr/lib/python2.7/site-packages/leapp/repository/actor_definition.py", line 72, in _do_run
Jan 26 10:12:11 localhost upgrade[663]:     actor_instance.run(*args, **kwargs)
Jan 26 10:12:11 localhost upgrade[663]:   File "/usr/lib/python2.7/site-packages/leapp/actors/__init__.py", line 335, in run
Jan 26 10:12:11 localhost upgrade[663]:     self.process(*args)
Jan 26 10:12:11 localhost upgrade[663]:   File "/usr/share/leapp-repository/repositories/system_upgrade/el7toel8/actors/removeupgradebootentry/actor.py", line 20, in process
Jan 26 10:12:11 localhost upgrade[663]:     remove_boot_entry()
Jan 26 10:12:11 localhost upgrade[663]:   File "/usr/share/leapp-repository/repositories/system_upgrade/el7toel8/actors/removeupgradebootentry/libraries/removeupgradebootentry.py", line 41, in remove_boot_entry
Jan 26 10:12:11 localhost upgrade[663]:     '/bin/mount', '-a'
Jan 26 10:12:11 localhost upgrade[663]:   File "/usr/lib/python2.7/site-packages/leapp/libraries/stdlib/__init__.py", line 188, in run
Jan 26 10:12:11 localhost upgrade[663]:     result=result
Jan 26 10:12:11 localhost upgrade[663]: CalledProcessError: Command ['/bin/mount', '-a'] failed with exit code 32.
Jan 26 10:12:11 localhost upgrade[663]: ==========================================================================================================
Jan 26 10:12:11 localhost upgrade[663]: Actor remove_upgrade_boot_entry unexpectedly terminated with exit code: 1 - Please check the above details
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