bugfix: independently check the "disabled" field to avoid schema ve…

[membphis]

…rification

with other parameters.

What this PR does / why we need it:

fix #2084

=== TEST 4: get plugin node-status schema
--- request
GET /apisix/admin/plugins/node-status
--- response_body
{"properties":{"disable":{"type":"boolean"}},"additionalProperties":false,"type":"object"}
--- no_error_log
[error]



=== TEST 5: get plugin prometheus schema
--- request
GET /apisix/admin/plugins/prometheus
--- response_body
{"properties":{"disable":{"type":"boolean"}},"additionalProperties":false,"type":"object"}
--- no_error_log
[error]

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible?

[membphis]

@nic-chen do you have time to review this PR ?

[juzhiyuan]

LGTM