allow multiple roles in a single node

[kpacha]

Locate multiple services on the same node is a common practice. But the current implementation of the serf-master module does not support it. In order to allow this use-case, the 'role' tag could handle a list of roles.

serf agent -tag role="web:db" -event-handler /opt/your/script.py

So the event handler must extract that list of roles and then the proxy would be able to execute a collection of handlers instead of just one.

i'm not really comfortable with python, so don't hesitate to comment the proposed implementation :)

[landscape-bot]

Code quality remained the same when pulling 28beb1e on kpacha:allow_multiple_roles into 2990899 on garethr:master.

[landscape-bot]

Code quality remained the same when pulling 455c419 on kpacha:allow_multiple_roles into 2990899 on garethr:master.

[coveralls]

Coverage increased (+0.02%) to 99.19% when pulling 455c419 on kpacha:allow_multiple_roles into 2990899 on garethr:master.