Additionnal note to my last comment:
After downgrading to "gnome-keyring 3.10.1-1ubuntu4" (from "gnome-keyring 3.10.1-1ubuntu4.1"), loging out/in, it started working again: a terminal started form a '.desktop' file has the proper SSH_AUTH_SOCK variable.
Additionnal note to my last comment:
After downgrading to "gnome-keyring 3.10.1-1ubuntu4" (from "gnome-keyring 3.10.1- 1ubuntu4. 1"), loging out/in, it started working again: a terminal started form a '.desktop' file has the proper SSH_AUTH_SOCK variable.