CLEANUP: ssl/cli: use a local context for "set ssl cert"

CLEANUP: ssl/cli: use a local context for "set ssl cert"

The command doesn't really need any storage since there's only a parser,
but since it used this context, there might have been plans for extension,
so better continue with a persistent one. Only old_ckchs, new_ckchs, and
path were being used from the appctx's ssl context. There ones moved to
the local definition, and the two former ones were removed from the appctx
since not used anymore.