mgr/dashboard: clean-up tox and unit tests

mgr/dashboard: clean-up tox and unit tests

Refactor CMake add_tox_test to automatically add py27 and/or py3 to
provided toxenvs.

Refactor tox.ini:
- Remove requirements-{py27,py3}.txt, as python release dependant
packages can be handled with PEP 508 syntax.
- Remove develepment dependencies from requirements.
- Move pycodestyle settings to separate section.
- Add flake8 check and other checkers (rst, naming, etc). Some of them
are commented out for future clean-ups (Ceph trackers have been opened)
- Pycodestyle removed, as flake8 is a wrapper for pycodestyle.
- Add instafail plugin to report failures immediately
- Add timeout plugin to limit max run time (sometimes test_tasks hangs)
- Remove unused dependencies (lru_cache, pluggy)

Test and code linting fixes:
- Unused imports
- Fixes to HACKING.rst

Doc:
- Update HACKING.rst

Add conftest.py to mock imported modules (rados, rbd, cephfs), and mock
also rados Error and OSError Exceptions.

Fixes: https://tracker.ceph.com/issues/40487
Fixes: https://tracker.ceph.com/issues/41152
Signed-off-by: Ernesto Puerta <epuertat@redhat.com>