Cache list of slots only with explicit LHCbNightlyConf commit id

The following discussion from !77 (merged) should be addressed:

• @clemenci started a discussion:

  We should cache the list of slots only if we have an explicit commit id, otherwise we cannot use this function with a branch name.

  Note that currently the function is always invoked with an explicit commit id, but we may want to use a branch (explicit or implicit) when we address #44.