Do not evaluate variables of string parameters in the nodes' description

Set the value of the string parameters in the nodes' description as
the unevaluated variable instead of the evaluated variable. The
evaluation will be made later, when the nodes with such parameters
will be computed.

meshroom/nodes/aliceVision/CameraInit.py

@@ -159,7 +159,7 @@ The metadata needed are:
             name='sensorDatabase',
             label='Sensor Database',
             description='''Camera sensor width database path.''',
-            value=os.environ.get('ALICEVISION_SENSOR_DB', ''),
+            value='${ALICEVISION_SENSOR_DB}',
             uid=[],
         ),
         desc.FloatParam(

meshroom/nodes/aliceVision/CameraLocalization.py

@@ -125,7 +125,7 @@ class CameraLocalization(desc.CommandLineNode):
             name='voctree',
             label='Voctree',
             description='''[voctree] Filename for the vocabulary tree''',
-            value=os.environ.get('ALICEVISION_VOCTREE', ''),
+            value='${ALICEVISION_VOCTREE}',
             uid=[0],
             ),
         desc.File(

meshroom/nodes/aliceVision/CameraRigCalibration.py

@@ -109,7 +109,7 @@ class CameraRigCalibration(desc.CommandLineNode):
             name='voctree',
             label='Voctree',
             description='''[voctree] Filename for the vocabulary tree''',
-            value=os.environ.get('ALICEVISION_VOCTREE', ''),
+            value='${ALICEVISION_VOCTREE}',
             uid=[0],
             ),
         desc.File(

meshroom/nodes/aliceVision/CameraRigLocalization.py

@@ -116,7 +116,7 @@ class CameraRigLocalization(desc.CommandLineNode):
             name='voctree',
             label='Voctree',
             description='''[voctree] Filename for the vocabulary tree''',
-            value=os.environ.get('ALICEVISION_VOCTREE', ''),
+            value='${ALICEVISION_VOCTREE}',
             uid=[0],
             ),
         desc.File(

meshroom/nodes/aliceVision/ImageMatching.py

@@ -74,7 +74,7 @@ If images have known poses, use frustum intersection else use VocabularuTree.
             name='tree',
             label='Voc Tree: Tree',
             description='Input name for the vocabulary tree file.',
-            value=os.environ.get('ALICEVISION_VOCTREE', ''),
+            value='${ALICEVISION_VOCTREE}',
             uid=[],
             enabled=lambda node: 'VocabularyTree' in node.method.value,
         ),

meshroom/nodes/aliceVision/ImageMatchingMultiSfM.py

@@ -65,7 +65,7 @@ Thanks to this node, the FeatureMatching node will only compute the matches betw
             name='tree',
             label='Voc Tree: Tree',
             description='Input name for the vocabulary tree file.',
-            value=os.environ.get('ALICEVISION_VOCTREE', ''),
+            value='${ALICEVISION_VOCTREE}',
             uid=[],
             enabled=lambda node: 'VocabularyTree' in node.method.value,
         ),

meshroom/nodes/aliceVision/KeyframeSelection.py

@@ -95,14 +95,14 @@ You can extract frames at regular interval by configuring only the min/maxFrameS
             name='sensorDbPath',
             label='Sensor Db Path',
             description='''Camera sensor width database path.''',
-            value=os.environ.get('ALICEVISION_SENSOR_DB', ''),
+            value='${ALICEVISION_SENSOR_DB}',
             uid=[0],
         ),
         desc.File(
             name='voctreePath',
             label='Voctree Path',
             description='''Vocabulary tree path.''',
-            value=os.environ.get('ALICEVISION_VOCTREE', ''),
+            value='${ALICEVISION_VOCTREE}',
             uid=[0],
         ),
         desc.BoolParam(