refresh bearer token from file to avoid stale tokens when using mount…

[grosser]

…ed service account token

fixes #530

[grosser]

with rebase this is now super clean :D

[kjetijor]

@client = Kubeclient::Client.new(....) going straight to @client.api_valid? seems to not work with this, it'll end up missing any call to refresh_bearer_token_from_file.

[grosser]

fixed

[cben]

Code is neat 👍

About performance: you mentioned several options on #530 and this implements the last one:

• reload on every request (in def rest_client and def http_options) since it's pretty cheap and usually an in-memory file

So this will add several syscalls on every request. But only when config uses a token file, which is probably mainly used in-cluster.
I don't know how to judge if that's Good Enough, but OK let's start with it and optimize later if needed.