Add keyspace creation to deb postinstall

```
root@d380e9ae-55cc-4ce2-be30-3e1cc66ee5be:~# dpkg -i cyanite_0.1.0_all.deb 
Selecting previously unselected package cyanite.
(Reading database ... 54455 files and directories currently installed.)
Unpacking cyanite (from cyanite_0.1.0_all.deb) ...
Setting up cyanite (0.1.0) ...
Creating cassandra schema...
Processing triggers for ureadahead ...
root@d380e9ae-55cc-4ce2-be30-3e1cc66ee5be:~# cqlsh -k metric
Connected to Test Cluster at localhost:9160.
[cqlsh 4.1.1 | Cassandra 2.0.6 | CQL spec 3.1.1 | Thrift protocol 19.39.0]
Use HELP for help.
cqlsh:metric> SELECT path from metric limit 1;

(0 rows)

cqlsh:metric>
```