Add a common RunnerPrefix type

There are several fields that are common among some of the data
structures in garm. The RunnerPrefix is just one of them. Perhaps we
should move some of the rest in a common type and embed that into the
types that share those fields.

Signed-off-by: Gabriel Adrian Samfira <gsamfira@cloudbasesolutions.com>

database/sql/enterprise.go

@@ -145,7 +145,7 @@ func (s *sqlDatabase) CreateEnterprisePool(ctx context.Context, enterpriseID str
 		ProviderName:           param.ProviderName,
 		MaxRunners:             param.MaxRunners,
 		MinIdleRunners:         param.MinIdleRunners,
-		RunnerPrefix:           param.RunnerPrefix,
+		RunnerPrefix:           param.GetRunnerPrefix(),
 		Image:                  param.Image,
 		Flavor:                 param.Flavor,
 		OSType:                 param.OSType,