add single repo sync as CDS sync action

a CDS sync synchronizes all and everything which is not efficient if you just modifified/cloned/etc. a single repository. if this case you want to sync this particular repository only.

this request adds this feature by adding an optional parameter 'repoid' to cds sync:

$ pulp-admin cds sync -h
Usage: pulp-admin <options> cds sync <options>

Options:
  -h, --help           show this help message and exit
  --hostname=HOSTNAME  CDS hostname (required)
  --repoid=REPOID      repo identifier

without '--repoid' the behaviour is like the current one, synchronize all and everyting. with '--repoid' only the specified repo will by synchronized.