Bug: init wait loop not working with external etcd in entrypoint.sh

[jspaleta]

Looks like SENSU_HOST is hardwired in the init wait loop which is causing external etcd to fail to pass the init test.

Proposed fix:
Define new envvar ETCD_HOST and have it default to the SENSU_HOST value if not explicitly set in the environment.

[portertech]

Possibly related to sensu/sensu-enterprise-go#1255