fixing cache timeout setting

[sid88in]

the current v0.8.4 fails if the cache timeout setting on the UI is not provided in edit datasource settings view: http://hostname:port/druiddatasourcemodelview/list/

self.cache_timeout causes the exception. The new logic is as follows:

1. try to find the cache timeout value from the UI
2. If not then get the default value in the caravel_config.py
3. If the value is not defined in both config and the UI then set it to 0.

[landscape-bot]

Repository health decreased by 0.25% when pulling c8b76f1 on sid88in:fix_attribute_error into d885dd3 on airbnb:master.

• 3 new problems were found (including 0 errors and 1 code smell).
• No problems were fixed.

[coveralls]

Coverage decreased (-0.1%) to 71.078% when pulling c8b76f1 on sid88in:fix_attribute_error into d885dd3 on airbnb:master.

[landscape-bot]

Code quality remained the same when pulling e04f212 on sid88in:fix_attribute_error into d885dd3 on airbnb:master.

[coveralls]

Coverage decreased (-0.1%) to 71.047% when pulling e04f212 on sid88in:fix_attribute_error into d885dd3 on airbnb:master.

[mistercrunch]

Mmmh, some of that logic is already here:
https://github.com/sid88in/caravel/blob/fix_attribute_error/caravel/viz.py#L217

What kind of stack trace were you getting?

[sid88in]

@mistercrunch I have added my stack trace here: #207

[mistercrunch]

It's been addressed in another PR, thanks for the PR though!