Control the collection of lvm grains via config

lvm grain collection can take a long time on systems with a lot of
volumes and volume groups. On one server we measured ~3 minutes, which
is way too long for grains.

This change is backwards-compatible, leaving the lvm grain collection
enabled by default. Users with a lot of lvm volumes/volume groups can
disable these grains in the minion config by adding "lvm" to
disabled_grains:

disabled_grains:
   ...
   - lvm

changelog/63114.added.md

@@ -0,0 +1 @@
+Control the collection of lvm grains via disabled_grains.

salt/grains/lvm.py

@@ -1,5 +1,14 @@
 """
 Detect LVM Volumes
+
+.. versionchanged:: 3008
+
+To disable these grains add "lvm" to "disabled_grains" in the minion config.
+
+.. code-block:: yaml
+
+    disabled_grains:
+      - lvm
 """
 
 import logging
@@ -16,6 +25,12 @@
 
 log = logging.getLogger(__name__)
 
+__virtualname__ = "lvm"
+
+
+def __virtual__():
+    return __virtualname__ not in __opts__.get("disabled_grains", [])
+
 
 def lvm():
     """