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>
Make lxc.functions return the default lxcpath if /etc/lxc/lxc.conf doesn't provide one

Currently it returns the default path only if /etc/lxc/lxc.conf missing.
Since default lxc.conf doesn't contain lxcpath variable (this is at least the case in ubuntu) all tools fails if one doesn't give -P

caglar@qgq:~/Project/lxc/examples$ sudo /usr/bin/lxc-create -n test
lxc-create: no configuration path defined

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