Extract DRBD8ShowInfo class

This class parses the `drbdsetup show` output and represents it in an
easily accessible format. It got extracted so that 1) the DRBD8 class
can focus more on DRBD logic rather than parsing and 2) it's easier to
adapt the parser to new formats.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>