Use related endpoint for schedules if available, always

[AlanCoding]

with change:

tower-cli schedule modify --create-on-missing --job-template=1072 --enabled=false --rrule="DTSTART:20180718T155801Z RRULE:FREQ=MONTHLY;INTERVAL=1" --name=foo4 -v
*** DETAILS: Checking for an existing record. *********************************
GET http://localhost:8013/api/v2/job_templates/1072/schedules/
Params: [('name', u'foo4'), ('unified_job_template', 1072)]

*** DETAILS: Writing the record. **********************************************
POST http://localhost:8013/api/v2/job_templates/1072/schedules/
Data: {'enabled': False, 'unified_job_template': 1072, 'rrule': u'DTSTART:20180718T155801Z RRULE:FREQ=MONTHLY;INTERVAL=1', 'name': u'foo4'}

Resource changed.
== ==== ==================== ======= 
id name unified_job_template enabled 
== ==== ==================== ======= 
51 foo4                 1072   false
== ==== ==================== ======= 

without change:

tower-cli schedule modify --create-on-missing --job-template=1060 --enabled=false --rrule="DTSTART:20180718T155801Z RRULE:FREQ=MONTHLY;INTERVAL=1" --name=foo5 -v
*** DETAILS: Checking for an existing record. *********************************
GET http://localhost:8013/api/v2/schedules/
Params: [('name', u'foo5'), ('unified_job_template', 1060)]

*** DETAILS: Writing the record. **********************************************
POST http://localhost:8013/api/v2/schedules/
Data: {'enabled': False, 'unified_job_template': 1060, 'rrule': u'DTSTART:20180718T155801Z RRULE:FREQ=MONTHLY;INTERVAL=1', 'name': u'foo5'}

Error: The Tower server says you can't make a request with the POST method to that URL (http://localhost:8013/api/v2/schedules/).

Addresses #578, while also touching as little as possible here.

[coveralls]

Coverage remained the same at 63.827% when pulling 3af83a1 on AlanCoding:sch_mod into 9a22336 on ansible:master.

[coveralls]

Coverage remained the same at 63.827% when pulling 3af83a1 on AlanCoding:sch_mod into 9a22336 on ansible:master.