Datasource should validate the keys in the SQL proc file

[edmorley]

In https://bugzilla.mozilla.org/show_bug.cgi?id=1125464#c8 it was found that we were using the master_host for everything, even though we had thought we had specified read_host for the appropriate selects in the SQL proc file.

Unfortunately it turned out we were using the property 'host' rather than 'host_type'., eg:

"get_performance_series": {

    "sql":"SELECT `interval_seconds`, `series_signature`, `type`, `last_updated`, `blob`
           FROM `performance_series`
           WHERE `interval_seconds` = ? AND `series_signature` = ?",

    "host":"read_host"

},

This issue would have been much easier to spot, if Datasource validated the keys and rejected any that it did not recognise.

Alternatively, if that is not something you wish to do - having a way to opt out of the default_host_type would mean we could catch any cases where we were not explicitly passing the desired host_type (possibly a good feature in its own right).