Add default command as well

No need to compile graylog support (switched to docker gelf driver).
Even to add graylog support, installation can be done using by by
passing an env variable.

JIRA INFRA-981

All dependencies upgraded except:
- redis (3.0.1 not compatible with rq)
- WTForms

JIRA INFRA-684

Allow to send metrics to a zabbix server

JIRA INFRA-614 #action In Progress

JIRA INFRA-576 #action In Progress

- requirements-to-freeze.txt is used to list only the needed packages
- requirements.txt is used to build a reproducible environment
- requirements-dev.txt is for testing and building the documentation

- update conda image and all dependencies
- uwsgi now available in conda for Python 3.6 (no need to install gcc to
  install it via pip)
- add new dependency: factory_boy / pytest-factoryboy