Default to verifying splunk cert

[olivergondza]

The SSL cert verification is not performed by default, and needs to be turned on by system property - which is obscuring the need to do so at all.

This change flips the default without changing any functionality. When the flag is not provided, it verifies the cert unless -Dsplunkins.verifySSL=false is provided.

This was verified manually to actually trust self-signed certs before, while it rejects them afterwards.

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[olivergondza]

@fengxx, please take a look! Thanks!

[fengxx]

thanks, but toggle default SSL verification will cause problems

1. all existing users who are using self cert are affected
2. splunk default uses SplunkServerDefaultCert for CN name, which will not pass TLS hostname verification
3. new plugin install doesn't work out of box, users have to edit Jenkins startup arguments
   I created allow user config CA and tls #33 instead