Add dryad package

Add dryad package

Package dryad provides:
 * implementation of Dryad interface
 * utilities to manage Dryad and its users

Verification steps:
    // compile tests
    GOARCH=arm GOARM=7 GOOS=linux ginkgo build -cover dryad
    // copy dryad/dryad.test to device
    // run everything
    ./dryad.test -ginkgo.v
    // run without measurements
    ./dryad.test -ginkgo.v -ginkgo.skipMeasurements
    // run prepare only
    ./dyrad.test -ginkgo.focus "should prepare"
    // private key will be printed on stderr
    // it can be used to verify that SSH key is properly installed

Currently dryad is being run as root on muxpi. Future patches will
probably use sudo and require configured file in /etc/sudoers.d/
directory.

Change-Id: I095361ffd1f4b2b3fa5dfe2c000f960dd32886e2
Signed-off-by: Aleksander Mistewicz <a.mistewicz@samsung.com>