Alertmanager: Add title support VictorOps receiver

[oryband]

In #417 support was added for VictorOps (PagerDuty alertnative).
However the receiver doesn't support most fields listed in the docs, specificaly the entity_display_name field which serves as a title.

The result of this is the incidents created by Alertmanager have a random string as a title, forcing the user to open the incident page and search for the description body in order to understand what the incident it about.

The change should be pretty straight forward.

In addition, please allow to set the rest of the fields described in the docs above.

Thanks!

[fabxc]

This would certainly be accepted rather quickly. Would you like to give implementing this a shot?

[oryband]

@fabxc sorry but no, we have moved away from VictorOps, great service though.

[siavashs]

We are going to use VictorOps soon, so I will implement this.