Fix metadata validation in share api

Currently validation method _check_metadata_properties()
expects that metadata parameter is a dict which contains
string keys and values, but this assumption is incorrect,
because user can specify key without value or vice versa.

Change-Id: I5cc863bb89f986815a4f6c1addf593f2da62530f
Closes-Bug: #1392827
Merge "Fix metadata validation in share api"