Handle output from call_cmd better in maas_storage

[plars]

Description

I was looking at some other things with the maas device connector and recalled that there were some gaps in how output is handled from this call_cmd method in maas_storage. There are some failure modes that wouldn't produce the output you would expect, and some places where it should raise an exception instead. This should close those gaps.

I also noticed while testing this that the previous unit tests fail on python3.12 because there's a way it can get TypeError when trying to run the command with None in it. This handles that potential error better, and also adds a way to inject the node_info so that testing is easier

Resolved issues

Resolves #264

Documentation

N/A

Web service API changes

N/A

Tests

Lots of test improvements, including a way to inject node_info in MaasStorage.
+4% coverage of maas_storage with the additional unit tests added too

[nadzyah]

can be a one-line assignment, but you can keep it as is since it's a personal preference

Suggested change

	# Better support for testing
	if node_info is None:
	self.node_info = self._node_read()
	else:
	self.node_info = node_info
	# Better support for testing
	self.node_info = self._node_read() if node_info is None else node_info