Add client usage to etcd server's crt for grpc gateway into etcd

[mmerrill3]

From this thread (etcd-io/etcd#9785 (comment)), it appears there have been fixes in other projects to handle where etcd uses the server's crt internally when acting as a client.

I put some more info in this issue in kops: kubernetes/kops#8512

Basically, we see the following message in the etcd.log file, about every few minutes:

cat etcd.log |grep -A 1 rejected

2020-03-13 16:19:49.046488 I | embed: rejected connection from "127.0.0.1:25786" (error "tls: failed to verify client's certificate: x509: certificate specifies an incompatible key usage", ServerName "")
WARNING: 2020/03/13 16:19:49 grpc: addrConn.createTransport failed to connect to {0.0.0.0:4001 0  <nil>}. Err :connection error: desc = "transport: authentication handshake failed: remote error: tls: bad certificate". Reconnecting...
--
2020-03-13 16:22:01.577902 I | embed: rejected connection from "127.0.0.1:36272" (error "tls: failed to verify client's certificate: x509: certificate specifies an incompatible key usage", ServerName "")
WARNING: 2020/03/13 16:22:01 grpc: addrConn.createTransport failed to connect to {0.0.0.0:4001 0  <nil>}. Err :connection error: desc = "transport: authentication handshake failed: remote error: tls: bad certificate". Reconnecting...
--
2020-03-13 16:23:45.243584 I | embed: rejected connection from "127.0.0.1:44390" (error "tls: failed to verify client's certificate: x509: certificate specifies an incompatible key usage", ServerName "")
WARNING: 2020/03/13 16:23:45 grpc: addrConn.createTransport failed to connect to {0.0.0.0:4001 0  <nil>}. Err :connection error: desc = "transport: authentication handshake failed: remote error: tls: bad certificate". Reconnecting...

It appears the messages are benign, but they are much more prevalent in 3.4.3 than earlier versions of etcd (3.3.13).

We could add client usage to the server certificate in options.go when we make the GRPCServerConfig.

I am not sure why this is more visible in etcd 3.4.3 versus version 3.3.13.

[mmerrill3]

this is fixed.