mgr/vol: improvements and cleanup for "list_volumes()"

mgr/vol: improvements and cleanup for "list_volumes()"

List volumes returns a list of of 1-member dictionaries where each
member is "{'name': <volname>}". This format is useful printing output
of the command "ceph fs volume ls" but for internal use this format is
hinderance since before using it, this list needs to be resolved  from
list of dictionaries to list of strings.

Return a list of strings and move the code for converting it to list of
dictionaries to method "list_fs_volumes()" (which is method run by "ceph
fs volume ls" command).

This triggers change in async_job.py where we need volnames. The code
here converts list of dicts to list of string. This needs to removed.

Second, don't create unnecessary temporary variable "fs_map" in
"list_volumes()" since it is not required. To get fs names we can
directly iterate.

Signed-off-by: Rishabh Dave <ridave@redhat.com>