nsfarm/lxd: no longer require image file to be executable

It makes no sense as we reset access rights either way. Let's just check
if it is standard file and that is it.

imgs/README.md

@@ -3,12 +3,12 @@ Image definitions for NSFarm
 This directory contains definitions of images used in NSFarm. Images define
 content of container used during testing.
 
-Image is defined by existence of executable file `NAME.sh` where `NAME` is name
-for image used in NSFarm. Note that this is not name used in LXD (only part of
-it). This file definition suppose to be a bash script. It is run in container to
-prepare image. There is also a second part of image definition and that is
-directory named `NAME` here. Content of this directory is merged to container as
-it is.
+Image is defined by existence of file `NAME.sh` in `/imgs` directory where `NAME`
+is name for image used in NSFarm. Note that this is not name used in LXD (only
+part of it). This file definition suppose to be a bash script. It is run in
+container to prepare image. There is also a second part of image definition and
+that is directory named `NAME` here. Content of this directory is merged to
+container as it is.
 
 It is expected that `NAME.sh` file has on first line shebang and on second line
 name of base image to be used. It is suggested to continue with comment block with
@@ -30,7 +30,7 @@ This is a limitation of image name having to be a valid hostname.
 
 Base image
 ----------
-The base idea of images is that we can stack them on top of each other and that
+The base idea of images is that we can stack images on top of each other and that
 makes it easier to have common base and additional scripts that do only minimal
 changes to it. This concept is base image. In definition file `NAME.sh` this is
 sourced from second line. Leading hash is expected and stripped.

nsfarm/lxd/container.py

@@ -30,8 +30,6 @@ class Container():
         # Verify existence of image definition
         if not os.path.isfile(self._fpath):
             raise Exception("There seems to be no file describing image: {}".format(self._fpath))
-        if not os.access(self._fpath, os.X_OK):
-            raise Exception("The file describing image is not executable: {}".format(self._fpath))
         if not os.path.isdir(self._dpath):
             self._dpath = None
         # Make sure that we are connected to LXD