*: Compare strings by == or != instead of by `is` or `is not`

In python `is` checks object identity, not content:

    In [1]: a = 'hello'

    In [2]: a += ' world'

    In [3]: b = 'hello world'

    In [4]: id(a)
    Out[4]: 139735676540976

    In [5]: id(b)
    Out[5]: 139735675829040

    In [6]: a is b              <-- NOTE
    Out[6]: False

    In [7]: a == b              <-- NOTE
    Out[7]: True

So comparing strings by is is generally incorrect.

-> Fix strings comparision to use == / != everywhere (at least in found
places where string is compared wrt string literal)

We already had similar fix in a8526f4e, but seems the story continues
again.

/cc @alain.takoudjou, @Just1, @lu.xu, @jhuge, @tomo
/reviewed-by @jerome
/reviewed-on !122

slapos/promise/plugin/check_free_disk_space.py

@@ -265,7 +265,7 @@ class RunPromise(GenericPromise):
     check_date = self.getConfig('test-check-date')
     path = os.path.join(self.getPartitionFolder(), "") + "extrafolder"
     partitions = psutil.disk_partitions()
-    while path is not '/':
+    while path != '/':
       if not disk_partition:
         path = os.path.dirname(path)
       else:

slapos/promise/plugin/monitor_partition_space.py

@@ -190,7 +190,7 @@ class RunPromise(GenericPromise):
     check_date = self.getConfig('test-check-date')
     path = os.path.join(self.getPartitionFolder(), "") + "extrafolder"
     partitions = psutil.disk_partitions()
-    while path is not '/':
+    while path != '/':
       if not disk_partition:
         path = os.path.dirname(path)
       else:

slapos/test/promise/plugin/test_monitor_partition_space.py

@@ -43,7 +43,7 @@ class TestMonitorPartitionSpace(TestPromisePluginMixin):
     disk_partition = ""
     path = os.path.join(self.partition_dir, "") + "extrafolder"
     partitions = psutil.disk_partitions()
-    while path is not '/':
+    while path != '/':
       if not disk_partition:
         path = os.path.dirname(path)
       else: