auth/credentials: 'workgroup' set via command line will not drop existing ccache

auth/credentials: 'workgroup' set via command line will not drop existing ccache

The root cause for existing ccache being invalidated was use of global loadparm with
'workgroup' value set as if from command line. However, we don't really need to take
'workgroup' parameter value's nature into account when invalidating existing ccache.
When -U is used on the command line, one can specify a password to force ccache
invalidation.

The commit also reverts previous fix now that root cause is clear.