Support KUBECONFIG environment variable

[ark3]

One way of specifying an alternate context for kubectl is to set the environment variable KUBECONFIG to point to a non-default configuration file. Once done, this allows the user to use kubectl as always to talk to the desired cluster.

Telepresence does not respect this environment variable, leading to confusing situations where the telepresence log file shows a kubectl command failing whereas the same command typed at the shell completes successfully. Other (worse) failure modes are possible too.