fix grammar in some lxc mod err msgs: exists -> exist
avoid shadowing outer exists() func in lxc module

************* Module salt.modules.lxc
salt/modules/lxc.py:520: [W0621(redefined-outer-name), start] Redefining name 'exists' from outer scope (line 616)
salt/modules/lxc.py:555: [W0621(redefined-outer-name), stop] Redefining name 'exists' from outer scope (line 616)
Merge pull request #11057 from hulu/lxc-exists

shadowing & grammar in lxc module