RHEL7: "leapp upgrade" fails in reboot phase with message "OSError: [Errno 30] Read-only file system" then enters Emergency mode

Issue

• leapp upgrade fails in reboot phase with below error message and enters Emergency mode :

localhost upgrade[827]: Traceback (most recent call last):
localhost upgrade[827]:   File "/usr/bin/leapp", line 9, in <module>
localhost upgrade[827]:     load_entry_point('leapp==0.15.1', 'console_scripts', 'leapp')()
localhost upgrade[827]:   File "/usr/lib/python2.7/site-packages/leapp/cli/__init__.py", line 45, in main
localhost upgrade[827]:     cli.command.execute('leapp version {}'.format(VERSION))
localhost upgrade[827]:   File "/usr/lib/python2.7/site-packages/leapp/utils/clicmd.py", line 111, in execute
localhost upgrade[827]:     args.func(args)
localhost upgrade[827]:   File "/usr/lib/python2.7/site-packages/leapp/utils/clicmd.py", line 133, in called
localhost upgrade[827]:     self.target(args)
localhost upgrade[827]:   File "/usr/lib/python2.7/site-packages/leapp/cli/commands/upgrade/breadcrumbs.py", line 147, in wrapper
localhost upgrade[827]:     return f(*args, breadcrumbs=breadcrumbs, **kwargs)
localhost upgrade[827]:   File "/usr/lib/python2.7/site-packages/leapp/cli/commands/upgrade/__init__.py", line 89, in upgrade
localhost upgrade[827]:     repositories = util.load_repositories()
localhost upgrade[827]:   File "/usr/lib/python2.7/site-packages/leapp/cli/commands/upgrade/util.py", line 90, in load_repositories
localhost upgrade[827]:     manager.load()
localhost upgrade[827]:   File "/usr/lib/python2.7/site-packages/leapp/repository/manager.py", line 120, in load
localhost upgrade[827]:     repo.load(resolve=False, stage=_LoadStage.ACTORS, skip_actors_discovery=skip_actors_discovery)
localhost upgrade[827]:   File "/usr/lib/python2.7/site-packages/leapp/repository/__init__.py", line 153, in load
localhost upgrade[827]:     actor.discover()
localhost upgrade[827]:   File "/usr/lib/python2.7/site-packages/leapp/repository/actor_definition.py", line 207, in discover
localhost upgrade[827]:     q = Queue(1)
localhost upgrade[827]:   File "/usr/lib64/python2.7/multiprocessing/__init__.py", line 218, in Queue
localhost upgrade[827]:     return Queue(maxsize)
localhost upgrade[827]:   File "/usr/lib64/python2.7/multiprocessing/queues.py", line 63, in __init__
localhost upgrade[827]:     self._rlock = Lock()
localhost upgrade[827]:   File "/usr/lib64/python2.7/multiprocessing/synchronize.py", line 147, in __init__
localhost upgrade[827]:     SemLock.__init__(self, SEMAPHORE, 1, 1)
localhost upgrade[827]:   File "/usr/lib64/python2.7/multiprocessing/synchronize.py", line 75, in __init__
localhost upgrade[827]:     sl = self._semlock = _multiprocessing.SemLock(kind, value, maxvalue)
localhost upgrade[827]: OSError: [Errno 30] Read-only file system