Add optional label input

[robert-clegg-tessella]

Labels may be added to k8s pods; these can be specified in the manifest sent to the Spark Operator.

One benefit of this is that all pods with a certain label can be deleted. This would be useful when cleaning up after system tests.

Acceptance criteria

When submitting a job, the user has the option of including an input named "label" that has a string value.
Running the command kubectl get pods -l userLabel=my_label on a machine with kubectl configured should return only those pods for jobs with the label my_label.

Test Scenarios

• Test that labels can be defined for every pod available (all resource objects).
• Test that label names cannot be duplicated NA
• Test that label names are not case sensitive NA
• Test that label names are not restricted by max character limit (check the max boundry 256 char? and min boundry 1 char? limit)
• Test if special characters are accepted in label names, specially :, /, white space

[oliver-tarrant-tessella]

Add details in wiki for adding label and arguments

[oliver-tarrant-tessella]

Add details in wiki for adding label and arguments

Details added here: https://github.com/ukaea/piezo/wiki/WebAppUserGuide