Set empty alertmanager listen address in non-HA mode

[hamishforbes]

Set the alertmanager.cluster.listen-address CLI flag to an empty string when AM is not in HA mode.
Alertmanager tries to start clustering anyway, unless the flag is explicitly set as an empty string
https://github.com/prometheus/alertmanager#turn-off-high-availability

Causes alertmanager to crash on startup with the default non-HA cli args

level=debug ts=2021-07-02T04:01:32.241015977Z caller=cluster.go:172 component=cluster msg="resolved peers to following addresses" peers=
level=warn ts=2021-07-02T04:01:32.241506112Z caller=cluster.go:177 component=cluster err="couldn't deduce an advertise address: no private IP found, explicit advertise addr not provided"
level=error ts=2021-07-02T04:01:32.242211077Z caller=log.go:106 msg="error running cortex" err="
Failed to get final advertise address: No private IP address found, and explicit IP not provided
create memberlist
github.com/prometheus/alertmanager/cluster.Create
    /__w/cortex/cortex/vendor/github.com/prometheus/alertmanager/cluster/cluster.go:240
github.com/cortexproject/cortex/pkg/alertmanager.NewMultitenantAlertmanager
    /__w/cortex/cortex/pkg/alertmanager/multitenant.go:295
github.com/cortexproject/cortex/pkg/cortex.(*Cortex).initAlertManager
    /__w/cortex/cortex/pkg/cortex/modules.go:717
github.com/cortexproject/cortex/pkg/util/modules.(*Manager).initModule
    /__w/cortex/cortex/pkg/util/modules/modules.go:103
github.com/cortexproject/cortex/pkg/util/modules.(*Manager).InitModuleServices
    /__w/cortex/cortex/pkg/util/modules/modules.go:75
github.com/cortexproject/cortex/pkg/cortex.(*Cortex).Run
    /__w/cortex/cortex/pkg/cortex/cortex.go:390
main.main
    /__w/cortex/cortex/cmd/cortex/main.go:190
runtime.main
    /usr/local/go/src/runtime/proc.go:225
runtime.goexit
    /usr/local/go/src/runtime/asm_amd64.s:1371
unable to initialize gossip mesh
github.com/cortexproject/cortex/pkg/alertmanager.NewMultitenantAlertmanager
    /__w/cortex/cortex/pkg/alertmanager/multitenant.go:309
github.com/cortexproject/cortex/pkg/cortex.(*Cortex).initAlertManager
    /__w/cortex/cortex/pkg/cortex/modules.go:717
github.com/cortexproject/cortex/pkg/util/modules.(*Manager).initModule
    /__w/cortex/cortex/pkg/util/modules/modules.go:103
github.com/cortexproject/cortex/pkg/util/modules.(*Manager).InitModuleServices
    /__w/cortex/cortex/pkg/util/modules/modules.go:75
github.com/cortexproject/cortex/pkg/cortex.(*Cortex).Run
    /__w/cortex/cortex/pkg/cortex/cortex.go:390
main.main
    /__w/cortex/cortex/cmd/cortex/main.go:190
runtime.main
    /usr/local/go/src/runtime/proc.go:225
runtime.goexit
    /usr/local/go/src/runtime/asm_amd64.s:1371
error initialising module: alertmanager
github.com/cortexproject/cortex/pkg/util/modules.(*Manager).initModule
    /__w/cortex/cortex/pkg/util/modules/modules.go:105
github.com/cortexproject/cortex/pkg/util/modules.(*Manager).InitModuleServices
    /__w/cortex/cortex/pkg/util/modules/modules.go:75
github.com/cortexproject/cortex/pkg/cortex.(*Cortex).Run
    /__w/cortex/cortex/pkg/cortex/cortex.go:390
main.main
    /__w/cortex/cortex/cmd/cortex/main.go:190
runtime.main
    /usr/local/go/src/runtime/proc.go:225
runtime.goexit
    /usr/local/go/src/runtime/asm_amd64.s:1371
"

Also moves the added HA cli args to alertmanager_args rather than adding them directly to the container.
Thought it was a bit confusing to have args being set in multiple places and potentially args set in alertmanager_args being overridden.
Happy to remove that bit if there's a reason for it though!

[pracucci]

Both changes LGTM!

Also moves the added HA cli args to alertmanager_args rather than adding them directly to the container.
Thought it was a bit confusing to have args being set in multiple places and potentially args set in alertmanager_args being overridden.

It was because of legacy reasons. Not required anymore 👍