API shouldn't be calling create for already defined containers or destroy for non defined ones.

Currently it behaves like following which might be confusing for the code that checks the return value of those calls to determine whether operation completed successfully or not.

>> c = lxc.Container("r")
>>> c.create("ubuntu")
True
>>> c.create("ubuntu")
True
>>> c.create("ubuntu")
True
>>> c.create("ubuntu")
True
>>> c.create("ubuntu")
>>> c.destroy()
True
>>> c.destroy()
lxc-destroy: 'r' does not exist
False
>>> c.destroy()
lxc-destroy: 'r' does not exist
False

New behaviour

>>> c = lxc.Container("r")
>>> c.create('ubuntu')
True
>>> c.create('ubuntu')
False
>>> c.destroy()
True
>>> c.destroy()
False
>>>

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>