Fix installation error on Ubuntu 16.04

[vpetersson]

[...]
After this operation, 15.4 MB of additional disk space will be used.
Get:1 http://us-central1.gce.archive.ubuntu.com/ubuntu xenial/main amd64 libltdl7 amd64 2.4.6-0.1 [38.3 kB]
Get:2 http://us-central1.gce.archive.ubuntu.com/ubuntu xenial/universe amd64 python3-ply all 3.7-1 [46.2 kB]
Get:3 http://us-central1.gce.archive.ubuntu.com/ubuntu xenial/universe amd64 python3-pycparser all 2.14+dfsg-2build1 [58.0 kB]
    module = __import__(self.module_name, fromlist=['__name__'], level=0)
  File "/usr/lib/python3/dist-packages/agent/__init__.py", line 66, in <module>
    os.makedirs(BACKUPS_PATH, 0o711)
  File "/usr/lib/python3.5/os.py", line 241, in makedirs
    mkdir(name, mode)
FileExistsError: [Errno 17] File exists: '/opt/wott/backups'

[vpetersson]

I did not receive this every time, but I did receive it on one node, so let's improve our logic here.

[vpetersson]

Add lock-file or similar.

[rptrchv]

3 complexity points

[a-martynovich]

8