Fix panic when alertmanager applies configs

[jtlisi]

After rebasing #1686, I noticed a major bug in the Alertmanager. The notify.NewPipelineBuilder function is called every time a change is applied causing a panic:

--- FAIL: TestMultitenantAlertmanager_loadAllConfigs (0.02s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x16c72e6]

goroutine 15 [running]:
testing.tRunner.func1(0xc0001bca00)
	/usr/local/Cellar/go/1.13.6/libexec/src/testing/testing.go:874 +0x3a3
panic(0x1a2fbe0, 0x2440fe0)
	/usr/local/Cellar/go/1.13.6/libexec/src/runtime/panic.go:679 +0x1b2
github.com/prometheus/alertmanager/dispatch.(*Dispatcher).Stop(0x0)
	/Users/jtlisi/go/pkg/mod/github.com/prometheus/[email protected]/dispatch/dispatch.go:219 +0x26
github.com/cortexproject/cortex/pkg/alertmanager.(*Alertmanager).ApplyConfig(0xc00036b200, 0x1b69ac3, 0x5, 0xc00039a3f0, 0x6, 0x0)
	/Users/jtlisi/GrafanaLabs/projects/cortex/pkg/alertmanager/alertmanager.go:192 +0x263
github.com/cortexproject/cortex/pkg/alertmanager.(*MultitenantAlertmanager).newAlertmanager(0xc000091800, 0x1b69ac3, 0x5, 0xc00039a3f0, 0x0, 0x0, 0xc00051ba40)
	/Users/jtlisi/GrafanaLabs/projects/cortex/pkg/alertmanager/multitenant.go:434 +0x21e
github.com/cortexproject/cortex/pkg/alertmanager.(*MultitenantAlertmanager).setConfig(0xc000091800, 0x1b69ac3, 0x5, 0x1b9a6c1, 0x34, 0x247cd18, 0x0, 0x0, 0x501de00, 0x100)
	/Users/jtlisi/GrafanaLabs/projects/cortex/pkg/alertmanager/multitenant.go:401 +0xad1
github.com/cortexproject/cortex/pkg/alertmanager.(*MultitenantAlertmanager).syncConfigs(0xc000091800, 0xc00051b770)
	/Users/jtlisi/GrafanaLabs/projects/cortex/pkg/alertmanager/multitenant.go:289 +0x2cd
github.com/cortexproject/cortex/pkg/alertmanager.(*MultitenantAlertmanager).updateConfigs(0xc000091800, 0xc0003bc000, 0xc0000406c0)
	/Users/jtlisi/GrafanaLabs/projects/cortex/pkg/alertmanager/multitenant.go:271 +0x5c
github.com/cortexproject/cortex/pkg/alertmanager.TestMultitenantAlertmanager_loadAllConfigs(0xc0001bca00)
	/Users/jtlisi/GrafanaLabs/projects/cortex/pkg/alertmanager/multitenant_test.go:77 +0x4f8
testing.tRunner(0xc0001bca00, 0x1c646f0)
	/usr/local/Cellar/go/1.13.6/libexec/src/testing/testing.go:909 +0xc9
created by testing.(*T).Run
	/usr/local/Cellar/go/1.13.6/libexec/src/testing/testing.go:960 +0x350
FAIL	github.com/cortexproject/cortex/pkg/alertmanager	0.380s
FAIL
Error: Tests failed.

This PR fixes the issue and builds a single PipelineBuilder for each per-tenant alertmanager.

Checklist

• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[pracucci]

LGTM!