Allow for reinitialising of the TA cert

[Koenvh1]

Previously, once the TA cert was created, there was no way to reinitialise it. That meant that any changes regarding the rsync URI, HTTPS URI, validity, etc. could not be made. This PR adds a --force switch to allow the signer to be created again, provided the same key is used, as well as a convenient reinit shorthand.

koen@beta:~/Code/krill$ target/debug/krillta signer reinit --help
Reinitialise an already initialised signer

Usage: krillta signer reinit [OPTIONS] --proxy-id <path> --proxy-repository-contact <path> --tal-rsync <rsync URI> --private-key-pem <path>

Options:
  -i, --proxy-id <path>                   Path to the proxy ID JSON file
  -r, --proxy-repository-contact <path>   Path to the proxy repository contact JSON file
      --tal-rsync <rsync URI>             The rsync URI used for TA certificate on TAL and AIA
      --tal-https <HTTPS URI>             The HTTPS URI used for the TAL
      --private-key-pem <path>            The private key for the already initialised signer in PEM format
      --initial-manifest-number <number>  Set the manifest number [default: 1]
  -h, --help                              Print help