Enabling Flake8 and import sorting

Enabling flake8 verifications in tox.ini and adding import sorting.
All the plugins have been fixed for flake8 errors and the imports
have been sorted.

Change-Id: Icdefe9a877e891a683cc833315e4c3f94fffbab9
Signed-off-by: sousaedu <eduardo.sousa@canonical.com>