mgr/volumes: write volume metadata with shim class

mgr/volumes: write volume metadata with shim class

Add a class that works a bit like a python file object so that we
can simplify the flush function. Providing a file-like object to
the ConfigParser's write function avoids unnecessary copies to
a StringIO object and makes the code easier to read.

With no more uses of StringIO, the StringIO imports are removed.

Signed-off-by: John Mulligan <jmulligan@redhat.com>
(cherry picked from commit d06d8a1f5255b5da9106cd70acc0de99f89d2b96)