Make the chart work with a sqlite3 database #124

There are currently 2 issues that prevent using this chart to deploy gitea with a SQLite3 database. 1) The value from *gitea.config.database.HOST* is used to set *db.servicename* when all the databases under *gitea.database.buildIn* are not enabled. This causes a type error during the template processing: `Error: UPGRADE FAILED: template: gitea/templates/gitea/init.yaml:24:20: executing "gitea/templates/gitea/init.yaml" at <include "db.servicename" .>: error calling include: template: gitea/templates/_helpers.tpl:64:31: executing "db.servicename" at <.Values.gitea.config.database.HOST>: wrong type for value; expected string; got interface {}` 2) In *init_gitea.sh*, we use the value *db.servicename* and *db.port* to ping the database. If this database responds to ping, we proceed with the init. The problem here is that *db.port* is not set when all the databases under *gitea.database.buildIn* are disabled. In turn, this raises an error from busybox's *nc*, because no parameter is passed for *PORT*. This causes the init container to go in *CrashLoopBackOff* forever. The simple fix that is proposed in this PR is to check wether or not *.Values.gitea.config.database.DB_TYPE* is set to determine the value *db.servicename*. If *DB_TYPE* is *'sqlite3'*, leave *db.servicename* empty and use that to bypass the database ping.