Allow to get a disabled alarm

This change allow to get a disabled alarm.

To do this, it:
* add the ability to query the alarm-list API with the 'enabled' fields.
* change the default visibility of alarm-list API from enabled only to all.

Fixes bug #1231328

Change-Id: I80cd691696f7630d2ac9e9b2a69b5b0ae5ee7968
Don't load into alarms evaluators disabled alarms

With this patch, alarm evaluators query the ceilometer API with only
enabled alarms inteads of loading all alarms and skip the disabled one.

Related to bug #1231328

Change-Id: I7121c1d2703f2f82a7d910431696aa31cbf7fbe6
add more test cases to improve the test code coverage #3

In this patch, the following module test coverage improved to 100%

ceilometer/agent.py

Change-Id: I06ac05d8b3e77f67e0f25df08db25ddcb1f087ac
Merge "Allow to get a disabled alarm"
Merge "Don't load into alarms evaluators disabled alarms"
Merge "add more test cases to improve the test code coverage #3"