Add option to test if key is in git repo, raise error if not

Add option check_decoding_keys_in_git to Moore. In case the option is set, the configuration checks if the keys are in a git repo and raises an error if not.

The main use case would be for Online running where we want to prevent that we run with a configuration which is not available on cvmfs. To enable this check something like the following should be done in Online and Production scripts.

from PyConf.application import metainfo_repos, retrieve_encoding_dictionary
options.write_decoding_keys_to_git = False
metainfo_repos.global_bind(extra_central_tags=['commissioning']) # add commissioning branch
metainfo_repos.global_bind(repos=[]) # only use repos on cvmfs
retrieve_encoding_dictionary.global_bind(require_key_present=True) # require key is in repo
# optional
# options.require_specific_decoding_keys = ["b80bb34f", "cf39d86c"]