Description of problem:
Currently when scheduling an automation task via the APi the schedule_time is specified as a number of days in the future. Customers need to be able to specify and exact date and time for the automation task.

Version-Release number of selected component (if applicable):
5.11.0.18

How reproducible:
Always

Steps to Reproduce:
1. Use the API to create an automation request with a parameter called schedule_time with a date/time combination.

POST /api/automation_requests
{
  "uri_parts" : {
    "namespace" : "System",
    "class"     : "Request",
    "instance"  : "InspectME",
    "message"   : "create"
  },
  "parameters" : {
    "var1" : "value 1",
    "var2" : "value 2",
    "minimum_memory" : 2048,
    "schedule_time": "2019-08-14 17:41:06 UTC"
  },
  "requester" : {
    "auto_approve" : true
  }
}


Actual results:
scheduling fails

Expected results:
Automation task is scheduled at the requested date and time.

Additional info:
The UI allows scheduling an automation task at a specified date and time. The API should allow for the same functionality.

See BZ 1486765 introduced the functionality of scheduling automation tasks via the API.