Respect VIRTUAL_BMC for non-default config path

Adds the ability to override default configuration file location
by exporting the ``VIRTUALBMC_CONFIG`` variable, pointing to the
desired config file, into ``vbmcd`` and ``vbmc`` processes
environment.

Change-Id: I89f7cab09f4c0b2c1d2517c4b6f607ed291f235c

doc/source/user/index.rst

@@ -10,6 +10,11 @@ does all the heavy-lifting (speaks IPMI, calls libvirt) while ``vbmc``
 client is merely a command-line tool sending commands to the server and
 rendering responses to the user.
 
+Both tools can optionally consume optional configuration file, which
+defaults to ``$HOME/.vbmc/virtualbmc.conf``,
+``/etc/virtualbmc/virtualbmc.conf``, but can be overridden with the
+``VIRTUALBMC_CONFIG`` environment variable.
+
 You should set up your systemd to launch the ``vbmcd`` server on system
 start up or you can just run ``vbmcd`` from command line if you do not need
 the tool running persistently on the system. Once the server is up and

releasenotes/notes/add-config-env-8287bea486821653.yaml

@@ -0,0 +1,5 @@
+features:
+  - |
+    Adds the ability to override default configuration file location by
+    exporting the ``$VIRTUALBMC_CONFIG`` variable, pointing to the desired
+    config file, into ``vbmcd`` and ``vbmc`` processes environment.

virtualbmc/config.py

@@ -19,15 +19,13 @@ from virtualbmc import utils
 __all__ = ['get_config']
 
 _CONFIG_FILE_PATHS = (
+    os.environ.get('VIRTUALBMC_CONFIG', ''),
     os.path.join(os.path.expanduser('~'), '.vbmc', 'virtualbmc.conf'),
     '/etc/virtualbmc/virtualbmc.conf')
 
+CONFIG_FILE = next((x for x in _CONFIG_FILE_PATHS if os.path.exists(x)), '')
+
 CONFIG = None
-CONFIG_FILE = ''
-for config in _CONFIG_FILE_PATHS:
-    if os.path.exists(config):
-        CONFIG_FILE = config
-        break
 
 
 class VirtualBMCConfig(object):