add validation for job_gc_interval

[jazzyfresh]

Overview

follow-up to #5978

The command/agent currently panics if the duration is an invalid value, this pull request adds validation.

Behavior

The panic

panic: non-positive interval for NewTicker                                                                               [190/1172]
                                                                                                                                   
goroutine 188 [running]:                                                                                                           
time.NewTicker(0xfffffffdabf41c00, 0x240c218)                                                                                      
        /usr/local/go/src/time/tick.go:23 +0x190                                                                                   
github.com/hashicorp/nomad/nomad.(*Server).schedulePeriodic(0xc0000e2000, 0xc0004a6240)                                            
        /home/jasmine/go/src/github.com/hashicorp/nomad/nomad/leader.go:446 +0xe1                                                  
created by github.com/hashicorp/nomad/nomad.(*Server).establishLeadership                                                          
        /home/jasmine/go/src/github.com/hashicorp/nomad/nomad/leader.go:243 +0x3c8

The fix w/ command/agent config validation for job_gc_interval

[schmichael]

Great catch!

[notnoop]

Thanks! A small tweak suggestion too.

[github-actions]

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