Add a flag for the run command to read envs from file

testdata/.env

@@ -0,0 +1,22 @@
+PRIMARY=http://ph1.example.com|password
+REPLICAS=http://ph2.example.com|password
+FULL_SYNC=false
+
+CRON=* * * * *
+
+SYNC_CONFIG_DNS=true
+SYNC_CONFIG_DHCP=true
+SYNC_CONFIG_NTP=true
+SYNC_CONFIG_RESOLVER=true
+SYNC_CONFIG_DATABASE=true
+SYNC_CONFIG_MISC=true
+SYNC_CONFIG_DEBUG=true
+
+SYNC_GRAVITY_DHCP_LEASES=true
+SYNC_GRAVITY_GROUP=true
+SYNC_GRAVITY_AD_LIST=true
+SYNC_GRAVITY_AD_LIST_BY_GROUP=true
+SYNC_GRAVITY_DOMAIN_LIST=true
+SYNC_GRAVITY_DOMAIN_LIST_BY_GROUP=true
+SYNC_GRAVITY_CLIENT=true
+SYNC_GRAVITY_CLIENT_BY_GROUP=true

testdata/.gitignore

@@ -0,0 +1 @@
+!.env