Feature Request: Switch between multiple related hosts in a context

Description

The new context feature in CLI 19.03 makes it easy to switch between multiple Docker hosts and Kubernetes endpoints. I often deal with groups of Docker hosts in a cluster that share a common set of TLS cert and key. There isn't a convenient way to use a context, which has the cert and key, but switch between multiple hosts.

Steps to reproduce the issue:

1. Assume there is a context named node-1 that has a Docker endpoint at tcp://node-1:2376, ca, cert, and key defined.
2. Based on the node-1 context, attempt to connect to an endpoint at tcp://node-2:2376 using the same certs.

Describe the results you received:

Using the node-1 context, I attempted to directly create a node-2 context with a different endpoint. This didn't work because --docker cannot be used when creating a context from another.

$ docker context create --from node-1 --docker host=tcp://node-2:2376 node-2
cannot use --docker or --kubernetes flags when --from is set

I then created a node-2 context from node-1, and attempted to update just the endpoint address. This didn't work, because it seems the existing ca, cert, and key parameter values aren't kept.

$ docker context create --from psc-dev-gen node-2
$ docker context update --docker tcp://node-2:2376 node-2
$ docker context use node-2
$ docker info
Server:
ERROR: Get http://node-2:2376/v1.40/info: net/http: HTTP/1.x transport connection broken: malformed HTTP response "\x15\x03\x01\x00\x02\x02".
* Are you trying to connect to a TLS-enabled daemon without TLS?
errors pretty printing info

I also tried using the original context, but setting DOCKER_HOST. However, setting the DOCKER_HOST environment variable forces the context back to default.

$ docker context use node-1
node-1
Current context is now "node-1"
$ docker context ls --format 'table {{.Name}}\t{{.DockerEndpoint}}'
NAME                DOCKER ENDPOINT
default *           tcp://node-2:2376
node-1              tcp://node-1:2376
node-2              tcp://node-2:2376
Warning: DOCKER_HOST environment variable overrides the active context. To use a context, either set the global --context flag, or unset DOCKER_HOST environment variable.

Describe the results you expected:

Some way to use a context that has ca, cert, and key set, and be able to easily hop between a set of related Docker hosts. Using the pre-context method via environment variables, it was easy to set DOCKER_CONFIG to the directory with certs and DOCKER_TLS_VERIFY=1, then just change DOCKER_HOST to hop between hosts.

Additional information you deem important (e.g. issue happens only occasionally):

N/A

Output of docker version:

Client: Docker Engine - Community
 Version:           19.03.0-rc2
 API version:       1.40
 Go version:        go1.12.5
 Git commit:        f97efcc
 Built:             Wed Jun  5 01:37:12 2019
 OS/Arch:           linux/amd64
 Experimental:      false

Server: Docker Engine - Community
 Engine:
  Version:          19.03.0-beta3
  API version:      1.40 (minimum version 1.12)
  Go version:       go1.12.4
  Git commit:       c55e026
  Built:            Thu Apr 25 19:13:00 2019
  OS/Arch:          linux/amd64
  Experimental:     true
 containerd:
  Version:          v1.2.6
  GitCommit:        894b81a4b802e4eb2a91d1ce216b8817763c29fb
 runc:
  Version:          1.0.0-rc7+dev
  GitCommit:        029124da7af7360afa781a0234d1b083550f797c
 docker-init:
  Version:          0.18.0
  GitCommit:        fec3683

Output of docker info:

N/A

Additional environment details (AWS, VirtualBox, physical, etc.):

Start by tracing the CLI entry points for docker context create, docker context update, and context selection, then reproduce the shown commands with shared TLS settings. Done means a context can retain its ca, cert, and key while switching between related Docker host endpoints without falling back to the default context.