Instead of running tests with nosetests,
this commit migrates tox.ini to use ostestr
similar to other openstack projects.
Using ostestr, as well testrepository, results
in another useful feature where test results can be
seen in convenient graphical representation.
* added oslo_debug_helper
* extracted unit-like dependencies into external testenv,
that way they are not installed for target where
they are actually not required (like pep8)
* removed testenv:docs because monasca does not come
with sphinx documentation
* removed testenv:genconfig because monasca is not
compatible with supplying its entire configuration
they way oslo-config-generator expects