Adds Content-Type to alarm REST notifier

When doing a POST request in the REST alarm notifier,
we should include a Content-Type: 'application/json' header.

Conflicts:
	ceilometer/alarm/notifier/rest.py
	ceilometer/tests/alarm/test_notifier.py

Closes-Bug: #1346147
(cherry picked from commit 297a1b48183b476968164158584017e7a56ccf2b)
Change-Id: Ic02c8caeb294763443858dbbb3192e09c594c8cf
Merge "Adds Content-Type to alarm REST notifier" into stable/icehouse