Create alarms for discovered devices

Currently, we create checks for {sda,sdb,sdc}, but not every server will
have these devices.  This commit loops over {{ ansible_devices }}
instead so we create an alarm for any discovered device.

Additionally, if a user variable called maas_excluded_devices exists,
we'll skip over those devices when creating alarms.
Merge pull request #518 from mattt416/issue_430

Create alarms for discovered devices