Resolves #15835

Make MINIKUBE_HOME env variable usage more convenient with backward compatibility.

Current logic of MINIKUBE_HOME env usage isn't really obvious:

• If env var is not set, it uses ${HOME}/.minikube as default.
• If env var is set, it checks its basename (for some reason??):
  • If basename equals .minikube - use env var
  • Else it appends .minikube to env var value (for some reason??)

It's very implicit logic and goes against the conventional use of *_HOME env vars.

With this changes, the logic changes a bit:

• If env var is not set, it uses ${HOME}/.minikube as default (as it's now)
• If env var is set, it checks its basename (as it's now)
  • If basename equals .minikube - use env var (as it's now)
  • Else it checks existence of .minikube inside path, provided via env var (for backward compatibility)
    • If ${MINIKUBE_HOME}/.minikube exists - use this legacy path (for backward compatibility)
    • Else - use MINIKUBE_HOME as is (new)

So, here is example cases:

1. Env is unset:

   $ export MINIKUBE_HOME=""
   $ # run minikube
   $ ls -hAF ~ | grep minikube
   .minikube/
   $ ls -hAF ~/.minikube
   addons/   cache/    certs/    config/   files/    logs/     machines/
   

2. Env is set AND its basename is .minikube:

   $ export MINIKUBE_HOME="${XDG_DATA_HOME}/.minikube"
   $ # run minikube
   $ ls -hAF $XDG_DATA_HOME | grep minikube
   .minikube/
   $ ls -hAF $XDG_DATA_HOME/.minikube
   addons/   cache/    certs/    config/   files/    logs/     machines/
   

3. Env is set AND its basename is not .minikube AND there is NO .minikube inside:

   $ export MINIKUBE_HOME="${XDG_DATA_HOME}/minikube"
   $ ls -hAF $MINIKUBE_HOME
   $ # MINIKUBE_HOME is empty
   $ # run minikube
   $ ls -hAF $MINIKUBE_HOME
   addons/   cache/    certs/    config/   files/    logs/     machines/
   

4. Env is set AND its basename is not .minikube AND there is .minikube inside provided path:

   $ export MINIKUBE_HOME="${XDG_DATA_HOME}/minikube"
   $ ls -hAF $MINIKUBE_HOME
   .minikube/
   $ # MINIKUBE_HOME has state from previous version
   $ # run minikube
   $ ls -hAF $MINIKUBE_HOME
   .minikube/
   $ ls -hAF $MINIKUBE_HOME/.minikube
   addons/   cache/    certs/    config/   files/    logs/     machines/