Removed specific version in development docs

I suggest to remove the version number completely
for the following reasons:
1. Fuel version is tracked in the other repository
   at the moment (fuel-main), so every release would
   require modifications of conf.py file (which
   we we actually doing before), so can always be in
   a situation of 'outdated' development docs
2. It looks like there is no reason to handle versions
   of development documentation, which always must
   reflect the current state of master. Basically,
   saying that it is 4.0 in the middle of 4.1 release
   is wrong.

Closes-Bug: #1263642

Change-Id: Iac3f7afdf2a4bd1949d1d36444d32413ff5d169d
Merge "Removed specific version in development docs"