cgroups: don't fail parsing when colon not found

cgroups: don't fail parsing when colon not found

The format of cpuacct.stat seems to not always have a colon as separator.

It appears to *not* have the colon at least on debian's kernel 3.2.41 and
3.2.35, as well as on RHEL6 with 2.6.32. All of them have decent support for
cgroups/lxc, and the cgroups plugin fails to work with them without this patch.

Also, looking at other implementations shows that they *don't* expect a colon:
https://bitbucket.org/dotcloud/liblxcstats/src/2558b4fbbf589c609895b0badbfc7d413466d716/probes/cpuacct.c?at=default#cl-71
https://github.com/BrightcoveOS/Diamond/blob/master/src/collectors/cpuacct_cgroup/cpuacct_cgroup.py#L55