Truncate id hash to 10 chars

This helps avoid the sock path being longer than the max length allowed
by the kernel. See the following comment for more information:

https://github.com/saltstack/salt/issues/12172#issuecomment-43903643
Update event tests to use truncated hash
Merge pull request #12994 from terminalmage/issue12172

Truncate id hash to 10 chars