functions for setting and getting Elasticsearch index aliases

We want to have separate Elasticsearch indexes for each storage
device, but we want to be able to access the currently mounted
device by a single alias name.
These functions are for setting an alias and for getting the
target index for an alias.
sanitize Elasticsearch alias names; delete aliases before adding
fixed Elasticsearch index alias set and report functions
storage.mount_path returns '/' if no path provided

It's better than returning 'None'
Merge branch '171-docstore-index-alias'
fix cleanup functions for topics/facility data going into docstore