Add --pre flag for linkerd check command

[klingerf]

This branch updates linkerd check to accept a --pre flag, which will cause the command to run a subset of checks to validate that linkerd can be installed (fixes #1470). It will also validate that the linkerd namespace does not already exist (fixes #1473).

I made one additional change as part of this branch, which is to bypass the kubernetes server version check when running other linkerd subcommands. As it was previously written, those subcommands would preemptively fail if the kubernetes cluster did not meet the minimum required version. It seems better to let those commands run, and only warn in the check output if the minimum server version is not met.

Fixes #1470.
Fixes #1473.

[klingerf]

The pre-install check output looks like this:

$ ./bin/go-run cli check --pre --expected-version $(bin/root-tag)
kubernetes-api: can initialize the client..................................[ok]
kubernetes-api: can query the Kubernetes API...............................[ok]
kubernetes-api: is running the minimum Kubernetes API version..............[ok]
linkerd-api: control plane namespace does not already exist................[ok]
linkerd-version: can get the latest version................................[ok]
linkerd-version: cli is up-to-date.........................................[ok]

Status check results are [ok]

[rmars]

🐎 🐑 nice!!!