make BuildConfigFromConfigFile less sensitive

[tg123]

this PR is based on #50 and to fix #24, #25

1. user is not a necessary property now, it will not be checked if set. no user or user: "" are now acceptable.
2. current-context can be ignore if masterUrl is set. (see testcase NoCurrentContext)
3. http host will always skip validate the ca settings no master the value of insecure-skip-tls-verify
4. will throw a bad host exception is host is not set. (was a uri npe)
5. other cleanup for codes