Change how module is executed on the minions

[neilmunday]

Instead of invoking the inventory.audit function every few hours, implement "inventory.run" to execute every 10-15 minutes. This function should read the config files on the minion (see issue #21) to determine which functions need to be executed.

Proposed functions to run regularly:

• audit -> existing audit functionality minus logging user info
• user -> new function to report users currently logged into a minion
• metrics -> new function to report back metrics for the minion (e.g. CPU load, memory use, disk usage etc. see issue Add minion metric checks #16)

These functions may need to cache data on the minion which can then be used to decide whether to report back to the master or not. For example, if the users logged haven't changed then don't report back. This is similar to the existing audit cache.