mgr/tox: adding coverage target to the main tox.ini

mgr/tox: adding coverage target to the main tox.ini

Let's add a coverage target so we can measure this metric for mgr modules.

Usage examples:

Run coverage for a single module (percentages only):
  tox -e coverage -- cephadm
  tox -e coverage -- smb

Show uncovered line numbers:
  COVERAGE_REPORT=term-missing tox -e coverage -- cephadm

Generate a visual HTML report:
  COVERAGE_REPORT=html tox -e coverage -- cephadm
  firefox htmlcov/index.html

Signed-off-by: Redouane Kachach <rkachach@ibm.com>