Simplify conditional statements in slurm conf template

[hanwen-pcluste]

Uri is required by Database section. Host is required by ExternalSlurmdbd. Therefore, to check the existence of Uri or Host, we just need to check the existence of Database or ExternalSlurmdbd.

This improvement simplifies the code and reduces the burden of mocking in testing.

FYI: {% if variable %} is equivalent to {% if variable != None and variable is defined %}

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