modules.lxc.init(): rename exists=>does_exist to avoid shadowing

************* Module salt.modules.lxc
salt/modules/lxc.py:794: [W0621(redefined-outer-name), init] Redefining name 'exists' from outer scope (line 1469)
Merge pull request #13704 from hulu/lxc-shadow

modules.lxc.init(): rename exists=>does_exist to avoid shadowing