Allow validating candidate configuration

[Nothing4You]

useful for example when generating an xml configuration and uploading it

[mrichardson03]

Validation is an op command, and can be done with 'panos_op'.

[Nothing4You]

unfortunately that only returns a job id from an enqueued validation job.
unlike panos_commit_firewall there is no sync parameter for panos_op.
this seems to be a limitation of pan-os-python python though, so i guess it should be requested there instead?

it could probably still be done without explicit support for sync in panos_op in pan-os-python though by calling syncjob directly.