Increase share-network default quota

Now Manila has too small quota for share networks, that affects gate tests with
exception "ShareNetworksLimitExceeded".

Changes:
- increased quota from 5 to 10;
- updated description of quota with proper info.

Change-Id: I1e3127780785ec689d02a4a0bcce42d903df7d72
Closes-Bug: #1368460