lxcapi_create: split out the template execution

Make it its own function to make both more readable.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
lxcapi_create: fix template handling

1. If no template is passed in, then do not try to execute it.  The user
just wanted to write the configuration.

2. If template is passed in as a full path, then use that instead of
constructing '$templatedir/lxc-$template'.

Reported-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>