This simply isn't true. The whole "17 agencies" thing is a talking point that first came up in one of Hillary Clinton's debates and gets repeated without challenge.
The "17 agencies" didn't all independently make their own assesments about what happened and decided it was the Russians. Instead, James Clapper (at the time, the Director of National Intelligence) made the claim that the Russians were behind certain hacks. Clapper is the ultimate head of sixteen out of seventeen of the agencies.
The actual agencies involved include parts of the coast guard and the department of energy and other groups that seem REALLY unlikely to have conducted an in-depth, independent investigation into the hacking of a politician's emails.
Also, this is the same James Clapper who lied under oath to congress. Specifically, when asked “Does the NSA collect any type of data at all on millions or hundreds of millions of Americans?” He responded, “No, sir.” Wyden asked “It does not?” and Clapper said, “Not wittingly."
This is perjury and he should have been prosecuted for it. At a minimum, lying under oath makes it less likely that anything else he claims should be taken at face value.