Improving manual install docs

Reasons:
 - manual install docs are outdated;

Changes:
 - doc improvement;

Change-Id: I3580293eb92e9063b3989a852e8e80715a09156b
Closes-Bug: #1342697
Adding image build procedure docs

Reasons:
 - there's a huge lack of documentation related
   to image building procedure.

Changes:
 - heat-jeos (Oz) image building doc added

Change-Id: I05139291ffff40de25859040b9431eb8af0954ac
Closes-Bug: #1342697