As a developer, I want to override management attribute values using environment variables

EAP on the cloud relies on container environment variables to customize its configuration to the container platform.

It's difficult to change a management attribute from the EAP standalone configuration if the attribute does not provide a default value with an expression that can be resolved at runtime.

For example, if the user wants to update the cache-size attribute on the /subsystem=foo/resource=my-resource, the cache-size attribute must be configured with an expression and a default value:

cache-size="${my-resource-cache-size:1234}"

so that the user can use a system property (or an env var with EAP7-1728) to resolve the expression with a new value.
However if the attribute is not configured with an expression, the user has no way to simply modify it.

This RFE proposes to be able to override an attribute value in the management model using an environment variable.

For the same example above, the user could set an env var before starting EAP to change the value of the cache size with something like:

SUBSYSTEM_FOO_RESOURCE_MY_RESOURCE_CACHE_SIZE=5678

The mapping of the env var is to be determined but a simple mapping could be the path of the resource + the name of the attribute and then modified to use SCREAMING_SNAKE_CASE convention.

This RFE does not impact expression resolution as the content of the env var will be set before any resolution is performed by the management API.