ENH: Add _cmd_prefix class variable to CommandLine

[effigies]

This PR adds a _cmd_prefix which is a string that is prepended (with no space) to the command. This will permit users to add installation-specific prefixes, whether they be installation directories that are not in the path or idiosyncratic names that have been chosen by a package manager.

The idea is that users could do something like:

import nipype
nipype.interfaces.ants.AntsCommand._cmd_prefix = '/opt/ants/'

or

import nipype
nipype.interfaces.fsl.FSLCommand._cmd_prefix = 'fsl_'

Fixes #1415.

cc @satra @TheChymera

[effigies]

@satra If you want to have a look at this to make sure it's what you were thinking, that'd be great.

[satra]

in general this looks good.

[effigies]

@satra Testing found what I think you were thinking of, which was a which function. Can you have a quick look at this latest commit?

[satra]

ah - so this is where spaces in the prefix would break this. also in the line above (for executable name), it assumes that self.cmd is not a full path with spaces.

perhaps this may help:

if os.path.isdir(self._cmd_prefix):
    executable_name = self._cmd_prefix + self.cmd 
elif os.path.isfile(self._cmd_prefix):
    executable_name = self._cmd_prefix
else:
    prefix = ''
    for part in self._cmd_prefix.split():
        prefix = prefix + part
        if os.path.isfile(prefix):
            executable_name = prefix
            break
    else:
        raise ValueError('{} does not contain a valid path'.format(self._cmd_prefix))
cmd_path = which(executable_name, env=runtime.environ)

[effigies]

@satra I opted to go with shlex.split(prefix + cmd), which should be more reliable than any parsing strategy we choose.

[effigies]

This is ready, as far as I'm concerned.

[effigies]

@satra This is the only thing left to review for 1.0.