validating threshold_rule and combination_rule should according alarm's type

[Liusheng]

Threshold_rule or combination_rule must be set to an alarm object, and they cannot be set at the same time.
Alarm's type must be one of threshold and combination.
So if when we updating an alarm with Threshold_rule and combination type, the return is error,but the faultstring is:

'UnsetType' object has no attribute 'as_dict'

it is ambiguous.

threshold_rule and combination_rule should be validated according alarm's type.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/78587

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/78587
Committed: https://git.openstack.org/cgit/openstack/ceilometer/commit/?id=2c23169109c75ff3e387b22979f8330a1fae8a10
Submitter: Jenkins
Branch: master

commit 2c23169109c75ff3e387b22979f8330a1fae8a10
Author: liu-sheng <email address hidden>
Date: Thu Mar 6 19:56:32 2014 +0800

    Ensure alarm rule conform to alarm type

    Threshold_rule or combination_rule must be set to an Alarm object,
    and they cannot be set at the same time.
    Alarm's type must be one of threshold and combination.
    Threshold_rule and combination_rule should be validated according alarm's
    type.

    Change-Id: I97bf82defeaf0dc1a0e897e10886fd736361041d
    Closes-bug: #1288677