Currently, openmpi has there implicit dependencies, which it tries to find in the system directories (see _verbs_dir(), _mxm_dir(), and _tm_dir()). This PR makes those dependencies explicit and the users now can specify custom prefixes to those libraries in packages.yaml.

1. Function _verbs_dir() is removed. Instead, openmpi depends on rdma-core:

   depends_on('rdma-core', when='fabrics=verbs')`

   If the users want to use libraries from the system directories, they need to specify rdma-core as an external package in packages.yaml:

   packages:
     rdma-core:
       paths:
         rdma-core: /usr
         buildable: False

2. Function _mxm_dir() is removed. Instead, openmpi depends on a new dummy package mxm:

   depends_on('mxm', when='fabrics=mxm')

   mxm can't be installed with Spack (at least for now) and needs to be specified in packages.yaml:

   packages:
     mxm:
       paths:
         mxm: /opt/mellanox/mxm
         buildable: False

3. Function _tm_dir() is removed. Instead, openmpi depends on a new package pbspro openpbs:

   depends_on('openpbs', when='schedulers=tm')

   pbspro openpbs can be installed with Spack. It depends on a new package libical and a new virtual package sendmail provided be a new real package ssmtp. Although pbspro openpbs can be installed with Spack, I can't guarantee that it will be usable: I don't have experience with the package to test it properly. Additionally, its scripts do not seem to properly account for the case when the package is installed to a non-system directory. Therefore, I would recommend the users to specify it as an external package too:

   packages:
     openpbs:
       paths:
         openpbs: /opt/pbs
         buildable: False

4. Package lsf is updated to allow for:

   $ spack spec openmpi schedulers=lsf
   $ spack fetch lsf