utils: add init_cache function

init_cache() creates the default directory in which OpenGnsys stores
images when using any cache enabled transfer method.

As of this commit this folder must exist for tiptorrent.py to
work properly.

Subprocess Popen object inside tiptorrent.py use
'cwd' optional parameter like:

	cwd='/opt/opengnsys/cache/opt/opengnsys/images/'

This folder convention might change in the future.

src/utils/cache.py

@@ -68,3 +68,15 @@ def generate_cache_txt():
             return
         content = ','.join(files)
         write_cache_txt(content)
+
+
+def init_cache():
+    """
+    If a cache partition is present, creates the following directories
+    /opt/opengnsys/images. This is the default folder in which images
+    are stored when using tiptorrent-cache.
+    """
+    mountpoint = mount_cache()
+    if mountpoint:
+        logging.debug(f'Creating cache directory at {mountpoint}')
+        os.makedirs('/opt/opengnsys/cache/opt/opengnsys/images')