Volume size could be specified to create volume

1. Provide a "volume_size" config at VolumeGroup
2. Make size as optional prameter, modify all volumes_client to
read the size from CONf if size is None.
3. Modify the testcase: test_volume_create_get_update
_delete_from_image to call create_volume with no size
parameter specified.

Change-Id: I2897f6d6bd970f73867f56d9d23a768cafcbfd80
Closes-Bug: #1293885
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index 761a077..fe383bc 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -193,7 +193,7 @@
 # admin credentials are known. (boolean value)
 #allow_tenant_isolation=false
 
-# Valid secondary image reference to be used in tests. (string
+# Valid primary image reference to be used in tests. (string
 # value)
 #image_ref={$IMAGE_ID}
 
@@ -940,6 +940,10 @@
 # value)
 #disk_format=raw
 
+# Default size in GB for volumes created by volumes tests
+# (integer value)
+#volume_size=1
+
 
 [volume-feature-enabled]