Default to `INFO` logging if no log-level was specified

[asnare]

When starting a command via databricks labs, the log-level to use is provided in the arguments payload.

If the user does not specify a log level, originally the situation was:

• The Databricks CLI defaulted to warnings (and higher).
• Blueprint-based labs subcommands would default to INFO (and higher).

Databricks CLI v0.241.0 changed this (probably in databricks/cli#2213):

• Previously the log-level passed through was notset, and blueprint set a default log-level to INFO.
• As of v0.241.0, a log-level of warn is passed through by default and this is therefore set as the log-level instead of INFO.

If possible we should default to INFO, provided the user has not explicitly specified a log-level to use.