Vagrant: Make host_ip a variable set in ~/.zulip-vagrant-config.

Vagrantfile

@@ -27,6 +27,7 @@ Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
   # The Zulip development environment runs on 9991 on the guest.
   host_port = 9991
   http_proxy = https_proxy = no_proxy = ""
+  host_ip_addr = "127.0.0.1"
 
   config.vm.synced_folder ".", "/vagrant", disabled: true
   config.vm.synced_folder ".", "/srv/zulip"
@@ -42,11 +43,12 @@ Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
       when "HTTPS_PROXY"; https_proxy = value
       when "NO_PROXY"; no_proxy = value
       when "HOST_PORT"; host_port = value.to_i
+      when "HOST_IP_ADDR"; host_ip_addr = value
       end
     end
   end
 
-  config.vm.network "forwarded_port", guest: 9991, host: host_port, host_ip: "127.0.0.1"
+  config.vm.network "forwarded_port", guest: 9991, host: host_port, host_ip: host_ip_addr
 
   if Vagrant.has_plugin?("vagrant-proxyconf")
     if http_proxy != ""

docs/brief-install-vagrant-dev.md

@@ -105,3 +105,16 @@ HOST_PORT 9971
 
 (and halt and restart the Vagrant guest), then you would visit
 http://localhost:9971/ to connect to your development server.
+
+
+If you'd like to be able to connect to your development environment from other
+machines than the VM host, you can manually set the host IP address in the
+'~/.zulip-vagrant-config' file as well. For example, if you set:
+
+```
+HOST_IP_ADDR 0.0.0.0
+```
+
+(and restart the Vagrant guest), your host IP would be 0.0.0.0, a special value
+for the IP address that means any IP address can connect to your development server.
+

docs/dev-env-first-time-contributors.md

@@ -864,6 +864,18 @@ HOST_PORT 9971
 (and halt and restart the Vagrant guest), then you would visit
 http://localhost:9971/ to connect to your development server.
 
+If you'd like to be able to connect to your development environment from other
+machines than the VM host, you can manually set the host IP address in the
+'~/.zulip-vagrant-config' file as well. For example, if you set:
+
+```
+HOST_IP_ADDR 0.0.0.0
+```
+
+(and restart the Vagrant guest), your host IP would be 0.0.0.0, a special value
+for the IP address that means any IP address can connect to your development server.
+
+
 [cygwin-dl]: http://cygwin.com/
 [vagrant-dl]: https://www.vagrantup.com/downloads.html
 [vagrant-dl-win]: https://releases.hashicorp.com/vagrant/1.8.6/vagrant_1.8.6.msi