fix: Validate the existence of configured volume paths on an OS

[AppCrashExpress]

Description

This PR adds simple validation for existence of configured host_volume path on OS, where nomad config validate is run.

Testing & Reproduction steps

Without the fix, the validation of config:

data_dir  = "/var/lib/nomad"

client {
  enabled       = true

  host_volume "avolume" {
    path = "/tmp/avolume"
  }
}

Runs successfully whether the file exists or not:

$ nomad config validate /tmp/config.hcl    
WARNING: mTLS is not configured - Nomad is not secure without mTLS!
Configuration is valid!

With the patch, it results in an error:

$ ./nomad-build config validate /tmp/config.hcl
WARNING: mTLS is not configured - Nomad is not secure without mTLS!
stat /tmp/avolume: no such file or directory
Configuration is invalid

Links

Fixes: #16968

Contributor Checklist

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  Please also consider whether the change requires notes within the upgrade
  guide. If you would like help with the docs, tag the nomad-docs team in this PR.

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Changes to Security Controls

There are no changes to security controls.

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[AppCrashExpress]

Should we be able to set the same names here, really?

[jrasell]

@AppCrashExpress there is nothing stopping you setting the same string multiple times in table driven tests. That being said, the name "test" does not help identify the test being run here, so having a slightly more descriptive name would be good.

[AppCrashExpress]

Oh, no, I just thought that they must be unique in general. I have, however, changed the names of volumes just in case.