changed boot_wait and made it to a variable

vagrant.json

@@ -8,12 +8,13 @@
         "cpus": "2",
         "headless": "true",
         "atlas_token": "PLACEHOLDER",
-        "write_zeroes": ""
+        "write_zeroes": "",
+        "boot_wait": "30s"
     },
     "builders": [
         {
             "type": "virtualbox-iso",
-            "boot_wait": "10s",
+            "boot_wait": "{{user `boot_wait`}}",
             "http_directory": "http",
             "disk_size": "{{user `disk_size`}}",
             "guest_os_type": "ArchLinux_64",
@@ -48,7 +49,7 @@
             ]
         }, {
             "type": "qemu",
-            "boot_wait": "10s",
+            "boot_wait": "{{user `boot_wait`}}",
             "http_directory": "http",
             "disk_size": "{{user `disk_size`}}",
             "iso_checksum_url": "{{user `iso_checksum_url`}}",
@@ -77,7 +78,7 @@
             ]
         }, {
             "type": "vmware-iso",
-            "boot_wait": "10s",
+            "boot_wait": "{{user `boot_wait`}}",
             "http_directory": "http",
             "disk_size": "{{user `disk_size`}}",
             "iso_checksum_url": "{{user `iso_checksum_url`}}",