Improve help strings

Follow oslo.config style guide for help strings better to create
consistent help strings:
* Capitalize first word of each help string
* Finish help strings with "."
* Improve wording and capitalization

Change-Id: I461589e4d20be6085281962ebf6e99262a58c761
fix traceback when default_version is not in the database

This commit lets the right error be raised which ends up in the cli as
e.g:

Datastore version '5.5' cannot be found.

Change-Id: I636bf43d135b2143f2a3bd1878215d7ec270330e
Closes-Bug: #1278965
Merge "fix traceback when default_version is not in the database"
Merge "Improve help strings"