Packaging+pants: Add BUILD macros: ST2_PACKS_GROUP and ST2_SVC_USER

There is some legacy bits in st2-packages.git that attempt
to pull the packs group and system user from /etc/st2/st2.conf.
But, that code is not in use. Effectively, the user/group names
have been hard-coded.

Rather than preserve an unused install feature, the pants+nfpm based
system package build will use hard-coded group/user names.

Author: cognifloyd

pants-plugins/macros.py

@@ -116,6 +116,10 @@ def st2_component_python_distribution(**kwargs):
 # https://github.com/pex-tool/pex/blob/v2.1.137/pex/common.py#L39-L45
 MTIME = "1980-01-01T00:00:00Z"
 
+# These are used for system packages (rpm/deb)
+ST2_PACKS_GROUP = "st2packs"
+ST2_SVC_USER = "st2"
+
 
 def st2_pack_archive(**kwargs):
     """Create a makeself_archive using files from the given dependencies.
@@ -159,7 +163,7 @@ def st2_pack_archive(**kwargs):
             f"/opt/stackstorm/packs/{pack_name}",
             # reproducibility flags:
             "--tar-extra",  # extra tar args: '--arg=value' (equals delimited) space separated
-            f"--owner=root:0 --group=root:0 --mtime={MTIME} --exclude=LICENSE",
+            f"--owner=root --group={ST2_PACKS_GROUP} --mtime={MTIME} --exclude=LICENSE",
             "--packaging-date",
             MTIME,
         ),