change silence to elastalert_silence

[abhishekjiitr]

The index in elastalert is stored as elastalert_silence instead of silence. This is confusing if the user is looking for the exact index names which elastalert uses

[abhishekjiitr]

This is applicable to new ES versions i.e. ES 6+. Atleast, we can add some information that index names change with ES versions

[abhishekjiitr]

@Qmando, can you review?