add global scrape interval config option

[lucabello]

Issue

Closes #99.

[lucabello]

A typo breaks the configuration, just like any other config option we have; there's nothing we can do about that except asking Juju for config option validation.

This has the same problem that the discussion for Mimir HA has, where if we try to correct a typo we open up numerous problems:

• do we try to make an educated guess on what the user meant? too risky and unpredictable;
• do we not apply the config and keep the previous value? we'd need to store the config values, or they'd be lost in an upgrade

Unfortunately I believe there's no good solution except an upstream one :)