lxc/file: Use random auth creds if --no-auth and --auth-user flags not specified
lxc/file: Use random auth creds if --no-auth and --auth-user flags not specified

If --no-auth is specified then no authentication is required.
If --auth-user is specified then the specified user name is required, and a random password is still generated.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

lxc/lxd,hallyn/lxd,stgraber/lxd,hallyn/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd