add --pull flag for podman create&run

Requirement from https://github.com/containers/libpod/issues/3575#issuecomment-512238393

Added --pull for podman create and pull to match the newly added flag in docker CLI.
`missing`: default value, podman will pull the image if it does not exist in the local.
`always`: podman will always pull the image.
`never`: podman will never pull the image.

Signed-off-by: Qi Wang <qiwan@redhat.com>

pkg/util/utils.go

@@ -356,3 +356,32 @@ func OpenExclusiveFile(path string) (*os.File, error) {
 	}
 	return os.OpenFile(path, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0666)
 }
+
+// PullType whether to pull new image
+type PullType int
+
+const (
+	// PullImageAlways always try to pull new image when create or run
+	PullImageAlways PullType = iota
+	// PullImageMissing pulls image if it is not locally
+	PullImageMissing
+	// PullImageNever will never pull new image
+	PullImageNever
+)
+
+// ValidatePullType check if the pullType from CLI is valid and returns the valid enum type
+// if the value from CLI is invalid returns the error
+func ValidatePullType(pullType string) (PullType, error) {
+	switch pullType {
+	case "always":
+		return PullImageAlways, nil
+	case "missing":
+		return PullImageMissing, nil
+	case "never":
+		return PullImageNever, nil
+	case "":
+		return PullImageMissing, nil
+	default:
+		return PullImageMissing, errors.Errorf("invalid pull type %q", pullType)
+	}
+}