Fix update of containerinfra cluster template's labels

[pawcykca]

This PR fixes errors while updating cluster template's labels.

Current data format (request body) send to Magnum API

[
  {
    "op": "replace",
    "path": "/labels",
    "value": "boot_volume_size=5"
  }
]

causes error (respond body) like below

{
	"errors": [{
			"request_id": "",
			"code": "client",
			"status": 400,
			"title": "Couldn't apply patch '[{'path': '/labels', 'op': 'replace', 'value': 'boot_volume_size=5",
			"detail": "Couldn't apply patch '[{'path': '/labels', 'op': 'replace', 'value': 'boot_volume_size=5'}]'. Reason: invalid syntax (<unknown>, line 1)",
			"links": []
		}
	]
}

Valid data format:

[
  {
    "op": "replace",
    "path": "/labels",
    "value": "{'boot_volume_size':'5'}"
  }
]

[nikParasyr]

LGTM. Thanks