Add os-base.md

docs/drivers/os-base.md

+<!--[metadata]>
++++
+title = "Driver options and operating system defaults"
+description = "Identify active machines"
+keywords = ["machine, driver, base, operating system"]
+[menu.main]
+parent="smn_machine_drivers"
+weight=-1
++++
+<![end-metadata]-->
+
+# Driver options and operating system defaults
+
+When Docker Machine provisions containers on local network provider or with a
+remote, cloud provider such as Amazon Web Services, you must define both the
+driver for your provider and a base operating system. There are over 10
+supported drivers and a generic driver for adding machines for other providers.
+
+Each driver has a set of options specific to that provider.  These options
+provide information to machine such as connection credentials, ports, and so
+forth.  For example, to create an Azure machine:
+
+Grab your subscription ID from the portal, then run `docker-machine create` with
+these details:
+
+```bash
+$ docker-machine create -d azure --azure-subscription-id="SUB_ID" --azure-subscription-cert="mycert.pem" A-VERY-UNIQUE-NAME
+```
+
+To see a list of providers and review the options available to a provider, see
+the reference for that driver.
+
+In addition to the provider, you have the option of identifying a base operating
+system. It is an option because Docker Machine has defaults for both local and
+remote providers. For local providers such as VirtualBox, Fusion, Hyper-V, and
+so forth, the default base operating system is Boot2Docker. For cloud providers,
+the base operating system is the latest Ubuntu LTS the provider supports.
+
+| Operating System        | Version | Notes              |
+| ----------------------- | ------- | ------------------ |
+| Boot2Docker             | 1.5+    | default for local  |
+| Ubuntu                  | 12.04+  | default for remote |
+| RancherOS               | 0.3+    |                    |
+| Debian                  | 8.0+    | experimental       |
+| RedHat Enterprise Linux | 7.0+    | experimental       |
+| CentOS                  | 7+      | experimental       |
+| Fedora                  | 21+     | experimental       |
+
+To use a different base operating system on a remote provider, specify the
+provider's image flag and one of its available images. For example, to select a
+`debian-8-x64` image on DigitalOcean you would supply the
+`--digitalocean-image=debian-8-x64` flag.
+
+If you change the base image for a provider, you may also need to change
+the SSH user. For example, the default Red Hat AMI on EC2 expects the
+SSH user to be `ec2-user`, so you would have to specify this with
+`--amazonec2-ssh-user ec2-user`.