There are several commands which do not require to load config:
completion, help, secret, version. Hovewer, PersistentPreRun call
at root level forces airshipctl to load config anyway. Therefore,
we can observe unnecessary config load errors instead of actual result.
This patch moves config load to subcommand level where it is necessary.
Change-Id: I774a7d038c408e71688f7bf04f1cb87a2132a342
bc90c4f39e