ncbi_blast_plus: Add macro for specifing num threads.

Inject number of thread dynamically from Galaxy using recent GALAXY_SLOTS implementation.
ncbi_blast_plus: Replace repeated cheetah code related to input files with tool macro.
ncbi_blast_plus: Replace repeated cheetah code related to advanced options with tool macro.
ncbi_blast_plus: Update README to reflect GALAXY_SLOTS changes.
Remove explicit shared_inputs in parallelism definitions.

Since pull request 142, these are no longer needed - anything not split is assumed to be shared.

https://bitbucket.org/galaxy/galaxy-central/pull-request/142/small-task-splitting-simpliciation/diff

Merged 2013-04-04, so already in the stable release of Galaxy.