reset cached access tokens in api specs

this allows once-ler to work in api specs by
resetting the user access tokens cache in the
global before :each block

Change-Id: I580007a17129e3e4b96f6310113e1578b5e63544
Reviewed-on: https://gerrit.instructure.com/37553
Reviewed-by: Brian Palmer <brianp@instructure.com>
Reviewed-by: Jon Jensen <jon@instructure.com>
Product-Review: Jon Jensen <jon@instructure.com>
QA-Review: Jon Jensen <jon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>