Add charset param to nova::db::mysql

Allow to specify the database charset but still default to 'latin1' if
none is provided.

Remove nova::params::nova_db_charset. Both osfamily were using 'latin1'
since the update for Folsom in 76d4632.
Merge pull request #249 from mgagne/add_db_mysql_charset_param

Allow to set charset in nova::db::mysql