Previously, the lookup plugin passes all its keyword arguments to
credstash's `getSecret`; while this works for passing the standard
parameters (version, region and table), this does not allow passing
a dictionary of key-value pairs as `getSecret`'s context parameter.
Instead, pop `version`, `region` and `table` from `kwargs`, supplying
the default value if they are not defined, and pass the rest of the `kwargs`
as the `context` parameter.
Michel Alexandre Salim
Marius Gedminas
Toshio Kuratomi
Scott Cunningham