fn: size restricted tmpfs /tmp and read-only / support (#1012)

* fn: size restricted tmpfs /tmp and read-only / support

*) read-only Root Fs Support
*) removed CPUShares from docker API. This was unused.
*) docker.Prepare() refactoring
*) added docker.configureTmpFs() for size limited tmpfs on /tmp
*) tmpfs size support in routes and resource tracker
*) fix fn-test-utils to handle sparse files better in create file

* test typo fix

api/models/call.go

@@ -108,6 +108,9 @@ type Call struct {
 	// Hot function idle timeout in seconds before termination.
 	IdleTimeout int32 `json:"idle_timeout,omitempty" db:"-"`
 
+	// Tmpfs size in megabytes.
+	TmpFsSize uint32 `json:"tmpfs_size,omitempty" db:"-"`
+
 	// Memory is the amount of RAM this call is allocated.
 	Memory uint64 `json:"memory,omitempty" db:"-"`