Update IsEmpty logic in finalizeClientTemplateConfig

[DerekStrickland]

When parsing the new consul-template configuration parameters introduced in 1.2.4 , the finalizeClientTemplateConfig function failed to account for the pre-1.2.4 fields in its logic and set the ClientTemplateConfig to nil only if none of the new configuration options were set.

This PR:

• Updates that logic to check the pre-1.2.4 fields
• Adds negative tests to ensure configs without the new consul-template settings introduced in 1.2.4 can still parse.

Closes #11902

[tgross]

LGTM.

I'm going to nitpick the commit title though... aren't we adding more logic to the IsEmpty() method?

[shoenig]

LGTM

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