add openstack volume_size option

Adds an option to OpenStack, `volume_size` (int), which is the size of
the Block Storage service volume in GB.

builder/openstack/run_config.go

@@ -40,6 +40,7 @@ type RunConfig struct {
 	UseBlockStorageVolume  bool   `mapstructure:"use_blockstorage_volume"`
 	VolumeName             string `mapstructure:"volume_name"`
 	VolumeType             string `mapstructure:"volume_type"`
+	VolumeSize             int    `mapstructure:"volume_size"`
 	VolumeAvailabilityZone string `mapstructure:"volume_availability_zone"`
 
 	// Not really used, but here for BC

website/source/docs/builders/openstack.html.md

@@ -275,6 +275,11 @@ builder.
     isn't specified, the default enforced by your OpenStack cluster will be
     used.
 
+-   `volume_size` (int) - Size of the Block Storage service volume in GB. If this
+    isn't specified, it is set to source image min disk value (if set) or
+    calculated from the source image bytes size. Note that in some cases this
+    needs to be specified, if `use_blockstorage_volume` is true.
+
 -   `volume_availability_zone` (string) - Availability zone of the Block
     Storage service volume. If omitted, Compute instance availability zone will
     be used. If both of Compute instance and Block Storage volume availability