This PR updates the behavior of Tahoe.start to check for and (re-)apply connection-related grid-settings to the local Tahoe nodedir in the event that the grid was joined previously via a "cheat code" (or if its name happens to match that of any of the grid names declared in the configuration files found in gridsync/resources/providers...). This change is intended to address the general problem in which user's the local grid-configuration (and/or the grid-configuration declared in a Recovery Key) might become out-of-date or otherwise stop working (due, e.g., to irrecoverable server-failures). In such cases, updating Gridsync to a newer build/version that contains a newer/correct grid-configuration file (in gridsync/resources/providers) will result in the user's local grid-configuration becoming updated accordingly. Only configuration changes that relate to connecting to the storage grid in question (such as the introducer fURL, the erasure-coding parameters, and the contents of servers.yaml) will be applied; all other user preferences, etc. will remain intact.

More specifically though, this (in conjunction with #576) is intended to provide a means for allowing users of Least Authority's "HRO Cloud" to re-connect to the grid (whose old/current grid-configuration is presently broken...).