Moving optional RebuildKeyspace out of tests.

[alainjobart]

vttablet's ActionAgent has three startup methods:

1. real production tablet
2. vtcombo in-memory tablet
3. in-memory test tablet for unit tests
   The optional RebuildKeyspace when SrvKeyspace doesn't exist only needs
   to be done in case .1, not 2. and 3. vtcombo does an explicit rebuild
   after having fully created the keyspace (so we don't need the background
   intermediate ones that will fail and spam the logs anyway). in-memory
   tests don't ususally need the SrvKeyspace (and if they do, they could
   call it).

[michael-berlin]

LGTM