Check and update readme, and document environment variable overrides and their usage

[shawaj]

For example all of the ones defined here:

hm-diag/hw_diag/app.py

Lines 34 to 42 in 23c6698

	SENTRY_DSN = os.getenv('SENTRY_DIAG')
	DIAGNOSTICS_VERSION = os.getenv('DIAGNOSTICS_VERSION')
	BALENA_ID = os.getenv('BALENA_DEVICE_UUID')
	BALENA_APP = os.getenv('BALENA_APP_NAME')
	HEARTBEAT_INTERVAL_HOURS = float(os.getenv('HEARTBEAT_INTERVAL_HOURS', 24))
	SHIP_DIAG_INTERVAL_HOURS = float(os.getenv('SHIP_DIAG_INTERVAL_HOURS', 1))
	NETWORK_WATCHDOG_INTERVAL_HOURS = float(os.getenv('NETWORK_WATCHDOG_INTERVAL_HOURS', 1))
	NEBRAOS_MIGRATION_INTERVAL_HOURS = float(os.getenv('NEBRAOS_MIGRATION_INTERVAL_HOURS', 24))
	MIGRATION_TASK_DISABLED = os.getenv('NEBRA_CLOUD_MIGRATION_DISABLED', 'false').lower() == 'true'

And also here:

hm-diag/hw_diag/utilities/hardware.py

Line 216 in 00f492a

if os.getenv('SWARM_KEY_URI_OVERRIDE'):

There may be more elsewhere

[shawaj]

Related to #442

[pritamghanghas]

we should move all of them to a configuration module. There are too many and too spread out. Only downside is such a refactor will involve changing lot of test cases. But shouldn't be very hard to do. it will be easy to document and reference.