allow to specify a list of alertmanager servers + fix attribute name ...

[Issif]

…for cloud events

What type of PR is this?

Uncomment one (or more) /kind <> lines:

/kind bug

/kind cleanup

/kind design

/kind documentation

/kind failing-test

/kind feature

Any specific area of the project related to this PR?

Uncomment one (or more) /area <> lines:

/area build

/area config

/area outputs

/area tests

What this PR does / why we need it:

• Allow to set a list of AlertManager servers to where the payload must be sent, it's required when HA mode is enabled in AlertManager.
• remove the disallowed character used in the CloudEvent extension

Which issue(s) this PR fixes:

#1025
#1050

Fixes #

Special notes for your reviewer:

[poiana]

LGTM label has been added.

Git tree hash: c714e21c0be5020f907d14aa100c387c69891896

[poiana]

LGTM label has been added.

Git tree hash: c3692b5dff83d8c813f5dfcc05c83d01903fa07c

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Issif, leogr

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