lvmetad: use the disabled flag in commands

lvmetad: use the disabled flag in commands

Commands already check if the lvmetad token is valid,
and if not, they rescan devices to repopulate lvmetad
before running.  Now, in addition to checking the
lvmetad token, they also check if the lvmetad disabled
flag is set.  If so, they do not use the lvmetad cache
and revert to disk scanning.