cloud-init fails when fstab is missing

[Balint Reczey]

In LP: #1877078 /etc/fstab was removed from the LXD squashfs.

Appearently due to the missing fstab cloud-init.service fails in a newly started Groovy container:

$ lxc launch ubuntu-daily:groovy c-i-test
$ lxc shell c-i-test
root@c-i-test:~# cat /var/log/cloud-init.log

...
2020-07-06 14:57:59,606 - util.py[DEBUG]: Reading from /etc/fstab (quiet=False)
2020-07-06 14:57:59,606 - handlers.py[DEBUG]: finish: init-network/config-mounts: FAIL: running config-mounts with frequency once-per-instance
2020-07-06 14:57:59,607 - util.py[WARNING]: Running module mounts (<module 'cloudinit.config.cc_mounts' from '/usr/lib/python3/dist-packages/cloudinit/config/cc_mounts.py'>) failed
2020-07-06 14:57:59,607 - util.py[DEBUG]: Running module mounts (<module 'cloudinit.config.cc_mounts' from '/usr/lib/python3/dist-packages/cloudinit/config/cc_mounts.py'>) failed
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/cloudinit/stages.py", line 848, in _run_modules
    ran, _r = cc.run(run_name, mod.handle, func_args,
  File "/usr/lib/python3/dist-packages/cloudinit/cloud.py", line 54, in run
    return self._runners.run(name, functor, args, freq, clear_on_fail)
  File "/usr/lib/python3/dist-packages/cloudinit/helpers.py", line 185, in run
    results = functor(*args)
  File "/usr/lib/python3/dist-packages/cloudinit/config/cc_mounts.py", line 381, in handle
    for line in util.load_file(FSTAB_PATH).splitlines():
  File "/usr/lib/python3/dist-packages/cloudinit/util.py", line 1422, in load_file
    with open(fname, 'rb') as ifh:
FileNotFoundError: [Errno 2] No such file or directory: '/etc/fstab'
...