Add share_size config option

All tempest tests create shares with fixed size, 1GB. This patch add
a config option in order to allow configurable share sizes as cinder
does (Cinder change-ID reference:
I2897f6d6bd970f73867f56d9d23a768cafcbfd80).

TrivialFix

Change-Id: Ifd09519b63c6c1a0ec8449e5dae1ffe0caa1b879
diff --git a/manila_tempest_tests/config.py b/manila_tempest_tests/config.py
index 8cf618a..9d0ed83 100644
--- a/manila_tempest_tests/config.py
+++ b/manila_tempest_tests/config.py
@@ -197,4 +197,7 @@
                default='none',
                choices=['none', 'writable', 'readable', 'dr'],
                help="Specify the replication type supported by the backend."),
+    cfg.IntOpt("share_size",
+               default=1,
+               help="Default size in GB for shares created by share tests."),
 ]