refactor(libcontainer): rename to CapabilitiesMask

The Capabilities field on libcontainer is actually used as a mask.
Rename the field so that this is more clear.

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
Upstream-commit: 128381e0f0372f10f88a847087aa91a972770c4b
Component: engine

components/engine/pkg/libcontainer/README.md

@@ -40,7 +40,7 @@ Sample `container.json` file:
       "HOSTNAME=11bb30683fb0",
       "TERM=xterm"
    ],
-   "capabilities" : [
+   "capabilities_mask" : [
       "SETPCAP",
       "SYS_MODULE",
       "SYS_RAWIO",