* Findings "speak for themselves" -U.N. spokesman
* Russia denounces findings as preconceived
* "Professionally executed massacre by regime" -U.S. envoy
By Louis Charbonneau and Michelle Nichols
UNITED NATIONS, Sept 18 (Reuters) - The United Nations on Wednesday defended its report on a Syria gas attack that Russia deemed one-sided as envoys from the five big U.N. powers met for a ...