Cleanup defining plugin configuration

[mlaferrera]

• Allow plugin configurations to be in stoq.cfg
• Merge plugin_opts with plugin config

Plugin configuration order of precedence:

1. plugin options provided at instantiation of Stoq(plugin_opts=...)

2. plugin configuration in stoq.cfg

3. plugin_name.stoq

   Plugin configuration may be added as a new section in stoq.cfg. For example, to add worker_rules to the yarascan plugin in stoq.cfg:

[yarascan]
worker_rules = /new/path/rules.yar

When handling configuration options within a plugin (again, using yarascan as an example) it would change from:

if plugin_opts and "worker_rules" in plugin_opts:
        worker_ruleset = plugin_opts["worker_rules"]
elif config.has_option("options", "worker_rules"):
        worker_ruleset = config.get("options", "worker_rules")
else:
        worker_ruleset = None

to:

worker_ruleset = config.get("options", "worker_rules", fallback=None)

[coveralls]

Coverage decreased (-0.03%) to 95.095% when pulling 9544fd8 on plugin_opts_config into 1f9c013 on master.