Use a proper type for unmarshalling config yaml

It's not possible to use the upstream type from the Alertmanager
project completely, as it will obfuscate any secrets in the yaml that
it creates, which we don't want. See the following issue for more
info: prometheus/alertmanager#1985

Documentation/api.md

@@ -1062,7 +1062,7 @@ AlertmanagerConfigList is a list of AlertmanagerConfig.
 | groupInterval |  | string | false |
 | repeatInterval |  | string | false |
 | matchers |  | [][Matcher](#matcher) | false |
-| continue |  | *bool | false |
+| continue |  | bool | false |
 | routes |  | [][Route](#route) | false |
 
 [Back to TOC](#table-of-contents)

go.mod

@@ -22,6 +22,7 @@ require (
 	github.com/prometheus-operator/prometheus-operator/pkg/apis/monitoring v0.42.1
 	github.com/prometheus/alertmanager v0.21.0
 	github.com/prometheus/client_golang v1.7.1
+	github.com/prometheus/common v0.13.0
 	github.com/prometheus/prometheus v1.8.2-0.20200907175821-8219b442c864
 	github.com/stretchr/testify v1.5.1
 	github.com/thanos-io/thanos v0.13.1-0.20200910143741-e0b7f7b32e9c

go.sum

@@ -358,7 +358,6 @@ github.com/go-openapi/spec v0.18.0/go.mod h1:XkF/MOi14NmjsfZ8VtAKf8pIlbZzyoTvZsd
 github.com/go-openapi/spec v0.19.2/go.mod h1:sCxk3jxKgioEJikev4fgkNmwS+3kuYdJtcsZsD5zxMY=
 github.com/go-openapi/spec v0.19.3/go.mod h1:FpwSN1ksY1eteniUU7X0N/BgJ7a4WvBFVA8Lj9mJglo=
 github.com/go-openapi/spec v0.19.6/go.mod h1:Hm2Jr4jv8G1ciIAo+frC/Ft+rR2kQDh8JHKHb3gWUSk=
-github.com/go-openapi/spec v0.19.7 h1:0xWSeMd35y5avQAThZR2PkEuqSosoS5t6gDH4L8n11M=
 github.com/go-openapi/spec v0.19.7/go.mod h1:Hm2Jr4jv8G1ciIAo+frC/Ft+rR2kQDh8JHKHb3gWUSk=
 github.com/go-openapi/spec v0.19.8 h1:qAdZLh1r6QF/hI/gTq+TJTvsQUodZsM7KLqkAJdiJNg=
 github.com/go-openapi/spec v0.19.8/go.mod h1:Hm2Jr4jv8G1ciIAo+frC/Ft+rR2kQDh8JHKHb3gWUSk=
@@ -462,9 +461,7 @@ github.com/golang/protobuf v1.4.0-rc.1/go.mod h1:ceaxUfeHdC40wWswd/P6IGgMaK3YpKi
 github.com/golang/protobuf v1.4.0-rc.1.0.20200221234624-67d41d38c208/go.mod h1:xKAWHe0F5eneWXFV3EuXVDTCmh+JuBKY0li0aMyXATA=
 github.com/golang/protobuf v1.4.0-rc.2/go.mod h1:LlEzMj4AhA7rCAGe4KMBDvJI+AwstrUpVNzEA03Pprs=
 github.com/golang/protobuf v1.4.0-rc.4.0.20200313231945-b860323f09d0/go.mod h1:WU3c8KckQ9AFe+yFwt9sWVRKCVIyN9cPHBJSNnbL67w=
-github.com/golang/protobuf v1.4.0 h1:oOuy+ugB+P/kBdUnG5QaMXSIyJ1q38wWSojYCb3z5VQ=
 github.com/golang/protobuf v1.4.0/go.mod h1:jodUvKwWbYaEsadDk5Fwe5c77LiNKVO9IDvqG2KuDX0=
-github.com/golang/protobuf v1.4.1 h1:ZFgWrT+bLgsYPirOnRfKLYJLvssAegOj/hgyMFdJZe0=
 github.com/golang/protobuf v1.4.1/go.mod h1:U8fpvMrcmy5pZrNK1lt4xCsGvpyWQ/VVv6QDs8UjoX8=
 github.com/golang/protobuf v1.4.2 h1:+Z5KGCizgyZCbGh1KZqA0fcLLkwbsjIzS4aV2v7wJX0=
 github.com/golang/protobuf v1.4.2/go.mod h1:oDoupMAO8OvCJWAcko0GGGIgR6R6ocIYbsSw735rRwI=
@@ -480,7 +477,6 @@ github.com/google/flatbuffers v1.11.0/go.mod h1:1AeVuKshWv4vARoZatz6mlQ0JxURH0Kv
 github.com/google/go-cmp v0.2.0/go.mod h1:oXzfMopK8JAjlY9xF4vHSVASa0yLyX7SntLO5aqRK0M=
 github.com/google/go-cmp v0.3.0/go.mod h1:8QqcDgzrUqlUb/G2PQTWiueGozuR1884gddMywk6iLU=
 github.com/google/go-cmp v0.3.1/go.mod h1:8QqcDgzrUqlUb/G2PQTWiueGozuR1884gddMywk6iLU=
-github.com/google/go-cmp v0.4.0 h1:xsAVV57WRhGj6kEIi8ReJzQlHHqcBYCElAvkovg3B/4=
 github.com/google/go-cmp v0.4.0/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
 github.com/google/go-cmp v0.5.0 h1:/QaMHBdZ26BB3SSst0Iwl10Epc+xhTquomWX0oZEB6w=
 github.com/google/go-cmp v0.5.0/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=