bigtable: Add ability to create table with initial splits
bigtable: Add ability to create table with initial splits

Note that creating initial splits is not normally necessary.
If, however, a large amount of data must be imported into the
table right after creation then creating ~100's of splits
across the expected key space can be beneficial for performance
until Bigtable figures out how to split the load naturally.

Change-Id: I531f194dab87056b77e2b950e5bd0a174264fbb5
Reviewed-on: https://code-review.googlesource.com/11290
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

googleapis/cloud-bigtable-cbt-cli,googleapis/cloud-bigtable-cbt-cli