* cmd/search: Add support for search of the packages
with a specific label.
* review Search method of Repositories for permit
different search modes.
* labels are k/v attributes and could be matched
through label key (with HasLabel method) or through
regex that use "$key" + "=" + "$value"
* installer.LuetRepository is now installer.LuetSystemRepository,
a struct that extend config.LuetRepository
* config: system_repositories option is now "repositories".
This resolve viper issue.
* config: cache_repositories option is now "repetitors"
This resolve viper issue.
* cmd/*: Now use new config.LuetRepository
* cmd/search: now create local luet repository if database_engine
is equal to "boltdb"
* concurrency could be configured now from cmdline,
configuration file or LUET_GENERAL__CONCURRENCY env variable
* verbose option is now related to debug option
