v0.8.0-alpha.2: Fixed out-of-range index dereferencing error in release controller

This commit fixes a problem where a corresponding release strategy was
resolved twice: once for an actual execution and second time for
reporting. These resolutions were happening in distinct places: the 1st
one in strategy executor, the latter in release controller. As a result,
the release controller one was causing a panic as it was not taking into
account the updated logic of strategy resolution where an incumbent is
supposed to look ahead and use it's successor's strategy, and the index
validity was only happening in strategy executor, which was calculating
the desired strategy correctly. This commit is also moving things
around: strategy executor is being initialized with a specific strategy
and a pointer to the target step and Execute() step takes the sequence
of executable releases as arguments.

Signed-off-by: Oleg Sidorov <[email protected]>