Fixed inconsistency with flag names

Fixed inconsistency with flag names for solvePnP. 
The default value for the function lacks the CV_ prefix. The code checks against "ITERATIVE". The suggested values for the parameters *include* the prefix.
Even though the enum CV_ITERATIVE (+ CV_P3P, CV_EPNP) = ITERATIVE (& P3P, EPNP), lets show to the users only one of them.
Now the user sees only {ITERATIVE, P3P, EPNP}.
Merge pull request #2813 from 23pointsNorth:patch-2