Add ability to merge envars of the form NVIDIA_VISIBLE_DEVICES_*

This allows someone to (for example) pass the following environment
variables:

NVIDIA_VISIBLE_DEVICES_0="0,1"
NVIDIA_VISIBLE_DEVICES_1="2,3"
NVIDIA_VISIBLE_DEVICES_WHATEVER="4,5"

and have the nvidia-container-toolkit automatically merge these into:

NVIDIA_VISIBLE_DEVICES="0,1,2,3,4,5"

This is useful (for example) if the full list of devices comes
from multiple, disparate sources.

Note: This will override whatever the original value of
NVIDIA_VISIBLE_DEVICES was (*excluding* its original value) if it also
exists as an environment variable already. We exclude the original value
to ensure that we have a way to override the default value of
NVIDIA_VISIBLE_DEVICES set to "all" inside a container image.

Signed-off-by: Kevin Klues <kklues@nvidia.com>

config/config.toml.amzn

@@ -11,6 +11,7 @@ load-kmods = true
 #no-cgroups = false
 #user = "root:video"
 ldconfig = "@/sbin/ldconfig"
+#alpha-merge-visible-devices-envvars = false
 
 [nvidia-container-runtime]
 #debug = "/var/log/nvidia-container-runtime.log"

config/config.toml.centos

@@ -11,6 +11,7 @@ load-kmods = true
 #no-cgroups = false
 #user = "root:video"
 ldconfig = "@/sbin/ldconfig"
+#alpha-merge-visible-devices-envvars = false
 
 [nvidia-container-runtime]
 #debug = "/var/log/nvidia-container-runtime.log"

config/config.toml.debian

@@ -11,6 +11,7 @@ load-kmods = true
 #no-cgroups = false
 #user = "root:video"
 ldconfig = "@/sbin/ldconfig"
+#alpha-merge-visible-devices-envvars = false
 
 [nvidia-container-runtime]
 #debug = "/var/log/nvidia-container-runtime.log"

config/config.toml.opensuse-leap

@@ -11,6 +11,7 @@ load-kmods = true
 #no-cgroups = false
 user = "root:video"
 ldconfig = "@/sbin/ldconfig"
+#alpha-merge-visible-devices-envvars = false
 
 [nvidia-container-runtime]
 #debug = "/var/log/nvidia-container-runtime.log"

config/config.toml.ubuntu

@@ -11,6 +11,7 @@ load-kmods = true
 #no-cgroups = false
 #user = "root:video"
 ldconfig = "@/sbin/ldconfig.real"
+#alpha-merge-visible-devices-envvars = false
 
 [nvidia-container-runtime]
 #debug = "/var/log/nvidia-container-runtime.log"