Allow disabling etcd3
Allow disabling etcd3

Etcd3 was enabled recently as new service in devstack [1]. But there's
no way to disable etcd3. This is required on architectures where no etcd
binaries are available (e.g. s390x). The long term goal of course should
be to have those binaries available. The short term circumvention is to
allow disabling the service in local.conf:

   disable_service etcd3

[1] https://github.com/openstack-dev/devstack/commit/546656fc0543ec2bc5b422fd9eee17f1b8122758
Change-Id: I6184ed193482dad9643ccb2b97133d4957485408
Partial-Bug: #1693192

nawawi/openstack,nawawi/openstack,noironetworks/devstack,richliu/devstack,richliu/devstack,openstack-dev/devstack,dtroyer/devstack,dtroyer/devstack,noironetworks/devstack,openstack-dev/devstack,group-policy/devstack,group-policy/devstack