Docs: reinforce warning against unshared storage

[stuclem]

Per vmware/vic-ui#72 (comment) we need to reinforce the warning in the docs about not using storage that is not available to all hosts in the cluster

[stuclem]

Added the following to the docs:

https://github.com/stuclem/vic-product/blob/vchui/docs/user_doc/vic_vsphere_admin/vch_storage.md

Storage Requirements and Limitations

The storage that you select for use as image and volume stores for VCHs must meet the following requirements.

• vSphere Integrated Containers Engine fully supports VMware vSAN datastores.

• vSphere Integrated Containers Engine supports all alphanumeric characters, hyphens, and underscores in datastore paths and datastore names, but no other special characters.

• Datastores that you specify as image and volume stores should ideally be accessible to all of the hosts in a cluster. Specifying storage that is not accessible to all of the hosts in a cluster is possible, but might result in all of your container VMs and container volumes being placed on the same host.

• If you specify different datastores in the different datastore options, and if no single host in a cluster can access all of the datastores, VCH deployment fails with an error.

  No single host can access all of the requested datastores.
  
  Installation cannot continue.

• If you specify different datastores in the different datastore options, and if only one host in a cluster can access all of them, VCH deployment succeeds with a warning.

  Only one host can access all of the image/container/volume datastores. This may be a point of contention/performance degradation and HA/DRS may not work as intended.

• VCHs do not support datastore name changes. If a datastore changes name after you have deployed a VCH that uses that datastore, that VCH will no longer function.

@zjs does this cover it?

[stuclem]

Added the following in the topic on the image store:

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If you are deploying the VCH to a vCenter Server cluster, the datastore that you designate as the image store must be shared by at least two, but preferably all, ESXi hosts in the cluster. Using non-shared datastores is possible, but limits the use of vSphere features such as vSphere vMotion® and VMware vSphere Distributed Resource Scheduler™ (DRS). Using non-shared datastores might lead to situations in which all container VMs and image files are stored on a single host.

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@zjs is this OK?

[stuclem]

Reviewed by @matthewavery and @anchal-agrawal in #1259.