[datadog_monitor] Fix crash for jmx integrations with init_config attributes set

[KSerrania]

What does this PR do?

Fixes a crash when the init_config attribute for a jmx integration (such asignite, jboss_wildfly) is set.

Previously, the init_config attribute would be merged in-place (through the merge! method) with another hash: { 'is_jmx': true, 'collect_default_metrics': true }. This would cause a crash if init_config is defined in attributes, because chef attributes cannot be modified that way:

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Error executing action `add` on resource 'datadog_monitor[jboss_wildfly]'
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Chef::Exceptions::ImmutableAttributeModification
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Node attributes are read-only when you do not specify which precedence level to set. To set an attribute use code like `node.default["key"] = "value"'

Cookbook Trace: (most recent call first)
----------------------------------------
/tmp/kitchen/cache/cookbooks/datadog/resources/monitor.rb:67:in `block (2 levels) in class_from_file'
/tmp/kitchen/cache/cookbooks/datadog/resources/monitor.rb:43:in `block in class_from_file'

This PR fixes this by not merging the attributes in-place. It also adds a kitchen test which exhibits the previous faulty behavior.