Pagerduty, fix incident key: use md5 hash from name and unique url

[z1nkum]

There are several cases old incident_key was not working well:

• warning and critical rules for same alert (non-resolved incident)
• NoData with nodata: critical

alert name + client_url looks unique enough
also we need hash because of length limit ("The maximum permitted length of this property is 255 characters" ref)

[coveralls]

Coverage decreased (-0.03%) to 65.0% when pulling d1fd4c3 on z1nkum:develop into c1f071e on klen:develop.