[QA] missing implementation of an existing occ command option

[mmattel]

Found during documenting the occ commands for the hsmdeamon,
option --import-masterkey is missing implementation (personal note, not a big effort implementing it)

Referencing:
#90 (comment) (Integrate HSM changes to encryption)
owncloud/docs#3724 (occ commands for HSM not documented)

Please note that the --export-masterkey has been implemented properly !
Both options are shown when running:

sudo -uwww-data ./occ encryption:hsmdaemon --help
Description:
  hsmdaemon tool

Usage:
  encryption:hsmdaemon [options]

Options:
      --export-masterkey                   export the private master key in base64
      --import-masterkey=IMPORT-MASTERKEY  import a base64 encoded private masterkey

@jnweiger @IljaN @butonic @micbar