Skip the profile manifest when the server libraries are absent

confluent_imgutil does not depend on confluent_server, and the yaml
import is optional too, yet both capture and pack dereference osimage
and yaml unconditionally when writing manifest.yaml.  With
confluent_osdeploy present but the server absent that raises rather
than producing a profile.

Guard the manifest on both being importable and say so, since rebase
is what the manifest exists for.  The yaml fallback now binds None
instead of leaving the name undefined.

The two call sites carried the manifest write verbatim in both, so fold
them into one function rather than duplicate the guard as well.

imgutil/imgutil

@@ -26,7 +26,7 @@ import time
 try:
     import yaml
 except ImportError:
-    pass
+    yaml = None
 path = os.path.dirname(os.path.realpath(__file__))
 path = os.path.realpath(os.path.join(path, '..', 'lib', 'python'))
 if path.startswith('/opt'):
@@ -197,6 +197,18 @@ def build_el_boot_tree(targpath):
     gather_bootloader(targpath)
 
 
+def write_manifest(outdir, indir):
+    if not (osimage and yaml):
+        sys.stderr.write('Warning: confluent server libraries unavailable, skipping manifest.yaml, '
+                         'osdeploy rebase will not work for this profile\n')
+        return
+    hmap = asyncio.run(osimage.get_hashes(outdir, indir))
+    with open('{0}/manifest.yaml'.format(outdir), 'w') as yout:
+        yout.write('# This manifest enables rebase to know original source of profile data and if any customizations have been done\n')
+        manifestdata = {'distdir': indir, 'disthashes': hmap}
+        yout.write(yaml.dump(manifestdata, default_flow_style=False))
+
+
 def capture_remote(args):
     targ = args.node
     outdir = args.profilename
@@ -267,11 +279,7 @@ def capture_remote(args):
     indir = '{}/profiles/default'.format(confdir)
     if os.path.exists(indir):
         copy_tree(indir, outdir)
-        hmap = asyncio.run(osimage.get_hashes(outdir, indir))
-        with open('{0}/manifest.yaml'.format(outdir), 'w') as yout:
-            yout.write('# This manifest enables rebase to know original source of profile data and if any customizations have been done\n')
-            manifestdata = {'distdir': indir, 'disthashes': hmap}
-            yout.write(yaml.dump(manifestdata, default_flow_style=False))
+        write_manifest(outdir, indir)
     label = '{0} {1} ({2})'.format(finfo['name'], finfo['version'], profname)
     with open(os.path.join(outdir, 'profile.yaml'), 'w') as profileout:
         profileout.write('label: {}\n'.format(label))
@@ -1672,11 +1680,7 @@ def pack_image(args):
             indir = '{}/profiles/default'.format(confdir)
             if os.path.exists(indir):
                 copy_tree(indir, outdir)
-                hmap = asyncio.run(osimage.get_hashes(outdir, indir))
-                with open('{0}/manifest.yaml'.format(outdir), 'w') as yout:
-                    yout.write('# This manifest enables rebase to know original source of profile data and if any customizations have been done\n')
-                    manifestdata = {'distdir': indir, 'disthashes': hmap}
-                    yout.write(yaml.dump(manifestdata, default_flow_style=False))
+                write_manifest(outdir, indir)
             tryupdate = True
     try:
         pwd.getpwnam('confluent')