Add read/write granularity to role based ACL

This patch adds 2 new configuration directive for ressource and item
ALLOWED_ROLES configuration parameter, global and per resource.
This allows for instance to require a role for writing but not for
reading.
Merge branch 'read-write-role-acl' of github.com:rs/eve into rs-read-write-role-acl
Olivier Poitrey
ALLOWED_[READ|WRITE]_ROLES
flake8 fixes.
Merge branch 'rs-read-write-role-acl' into develop