mgr/volumes: read volume metadata file using read_string

mgr/volumes: read volume metadata file using read_string

The read_string method, available in Python 3.2 (we assume Python 3.6 as
our current minimum python versino), supports parsing a provided string
for ini-style configuration parameters. Refactoring the reading of the
config file from cephfs into a simple iterator function and then
providing it to the ConfigParser as a single string, allows us to avoid
using StringIO and  simplifies the refresh function.

Signed-off-by: John Mulligan <jmulligan@redhat.com>
(cherry picked from commit dc1b4aac90beb82fd994ad7fa8ccf3b72b9936ac)