webadmin: Improve UI Plugin data reload logic

UI Plugin infra was designed to allow reloading plugin
meta-data and configuration without having to restart
Engine, i.e. refreshing WebAdmin in browser reloads the
plugin data automatically.

This patch improves existing data reload logic, comparing
cached data against (possibly missing) files, in addition
to comparing (possibly added or modified) files against
cached data.

Change-Id: Ibefec7f4a5748b89e762b88eafed044fab977d04
Bug-Url: https://bugzilla.redhat.com/996563
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>