Add support for not having koji volume mounted locally

With this patch, Pungi can be configured with a local directory to be
used as a cache for RPMs, and it will download packages from Koji over
HTTP instead of reading them from filesystem directly.

The files from the cache can then be hardlink as usual.

There is locking in place to avoid different composes running at the
same time to step on each other.

This is now supported for RPMs only, be it real builds or scratch
builds.

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

pungi/util.py

@@ -461,6 +461,9 @@ def get_volid(compose, arch, variant=None, disc_type=False, formats=None, **kwar
         if not variant_uid and "%(variant)s" in i:
             continue
         try:
+            # fmt: off
+            # Black wants to add a comma after kwargs, but that's not valid in
+            # Python 2.7
             args = get_format_substs(
                 compose,
                 variant=variant_uid,
@@ -472,6 +475,7 @@ def get_volid(compose, arch, variant=None, disc_type=False, formats=None, **kwar
                 base_product_version=base_product_version,
                 **kwargs
             )
+            # fmt: on
             volid = (i % args).format(**args)
         except KeyError as err:
             raise RuntimeError(
@@ -1146,3 +1150,16 @@ def read_json_file(file_path):
     """A helper function to read a JSON file."""
     with open(file_path) as f:
         return json.load(f)
+
+
+UNITS = ["", "Ki", "Mi", "Gi", "Ti"]
+
+
+def format_size(sz):
+    sz = float(sz)
+    unit = 0
+    while sz > 1024:
+        sz /= 1024
+        unit += 1
+
+    return "%.3g %sB" % (sz, UNITS[unit])