cloud/shepherd/equinix: implement initializer parallelism

This adds a flag which allows starting multiple initializers in
parallel, sharing the same SSH key/config and API rate limiting.

Change-Id: I415e855d9b649fac258e25d884cac17f895c91c0
Reviewed-on: https://review.monogon.dev/c/monogon/+/1135
Tested-by: Jenkins CI
Reviewed-by: Mateusz Zalega <mateusz@monogon.tech>
diff --git a/cloud/shepherd/equinix/manager/BUILD.bazel b/cloud/shepherd/equinix/manager/BUILD.bazel
index 004cd24..2c702fe 100644
--- a/cloud/shepherd/equinix/manager/BUILD.bazel
+++ b/cloud/shepherd/equinix/manager/BUILD.bazel
@@ -23,6 +23,7 @@
         "@io_k8s_klog_v2//:klog",
         "@org_golang_google_protobuf//proto",
         "@org_golang_x_crypto//ssh",
+        "@org_golang_x_sync//errgroup",
         "@org_golang_x_time//rate",
     ],
 )