bigtable/cmd/cbt: add versioning for cbt command
bigtable/cmd/cbt: add versioning for cbt command

This change adds version strings to the cbt command. They are set via
ldflags from the script we use to build CBT, so we can include the git
revision and current date.

Change-Id: Ia50d665142e0f97f50ec512747d30675548819b8
Reviewed-on: https://code-review.googlesource.com/11870
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Gary Elliott <d49662846a332c09d32ce16a761e8169b9defdba@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go