Fix convert integer CONCURRENT_REQUEST_PER_WORKER

[MichaelBitard]

Proposed changes:
If you try to override the variable in the docker-compose environment, it fails with:
CONCURRENT_REQUEST_PER_WORKER=20

[2021-07-02 09:35:53 +0000] [10] [ERROR] Exception in worker process
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/gunicorn/arbiter.py", line 589, in spawn_worker
    worker.init_process()
  File "/usr/local/lib/python3.7/site-packages/uvicorn/workers.py", line 64, in init_process
    super(UvicornWorker, self).init_process()
  File "/usr/local/lib/python3.7/site-packages/gunicorn/workers/base.py", line 134, in init_process
    self.load_wsgi()
  File "/usr/local/lib/python3.7/site-packages/gunicorn/workers/base.py", line 146, in load_wsgi
    self.wsgi = self.app.wsgi()
  File "/usr/local/lib/python3.7/site-packages/gunicorn/app/base.py", line 67, in wsgi
    self.callable = self.load()
  File "/usr/local/lib/python3.7/site-packages/gunicorn/app/wsgiapp.py", line 58, in load
    return self.load_wsgiapp()
  File "/usr/local/lib/python3.7/site-packages/gunicorn/app/wsgiapp.py", line 48, in load_wsgiapp
    return util.import_app(self.app_uri)
  File "/usr/local/lib/python3.7/site-packages/gunicorn/util.py", line 359, in import_app
    mod = importlib.import_module(module)
  File "/usr/local/lib/python3.7/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
  File "<frozen importlib._bootstrap>", line 983, in _find_and_load
  File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 728, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/home/user/rest_api/application.py", line 8, in <module>
    from rest_api.controller.router import router as api_router
  File "/home/user/rest_api/controller/router.py", line 3, in <module>
    from rest_api.controller import file_upload, search, feedback
  File "/home/user/rest_api/controller/search.py", line 47, in <module>
    concurrency_limiter = RequestLimiter(CONCURRENT_REQUEST_PER_WORKER)
  File "/home/user/rest_api/controller/utils.py", line 9, in __init__
    self.semaphore = Semaphore(limit - 1)
TypeError: unsupported operand type(s) for -: 'str' and 'int'
[

It should be converted to int beforehand

Status (please check what you already did):

• First draft (up for discussions & feedback)
• Final code
• Added tests
• Updated documentation

[MichaelBitard]

Related to #1228

[lalitpagaria]

LGTM!

[lalitpagaria]

@tholor can you please merge