buildstep.py: remove "ShellCommand" in LoggingBuildStep

some logging messages had "ShellCommand" hardcoded.

this commit writes the name of the class instead.

Signed-off-by: Ion Alberdi <ion.alberdi@intel.com>
buildstep: create ShellBaseStep

refactor what is necesary to execute a shell command
in a slave, in a mother class.

existing unit test from shellstep cover that refactoring.

Signed-off-by: Ion Alberdi <ion.alberdi@intel.com>
shellsequence: implement step

this steps permits one to launch multiple shell
commands in a single step. Each shell command
has its own optional stdio log, and haltOnFailure, etc parameter.
They however, all share the same workdir, use-pty, env parameters.

the skipping of commands can be driven by overriding
one method of that class (see doc).

Signed-off-by: Ion Alberdi <ion.alberdi@intel.com>
Merge yetanotherion/buildbot:listofshellcommands (PR #1045)
pep8, whitespace, etc.
rename 'cmd' to 'command' for consistency with ShellCommand
clarify log messages
revert to using _describe, but remove its docstring; see #2670
revert LoggingBuildStep and everything that depends on it, and merge the modifications to it into ShellBaseStep
edit the style section a bit
edit ShellSequence docs
Revert LoggingBuildStep, add CommandMixin and ShellMixin

These two mixin classes will be the new style for shell commands.
They're organized to be called from a `run` method, rather than
overriding lots of confusing hook methods like `createSummary` and
`evaluateCommand`.
refactor ShellSequence to use ShellMixin
refactor result calculations into results.py
update the customization docs to point to CommandMixin/ShellMixin
rename computeResultAndContinuation to computeResultAndTermination
docs tweaks suggested by @yetanotherion
Update comment

Let's be honest, I'm just testing GitHub's ability to edit PR's from the web..
Merge branch '9/shell-refactoring' of git://github.com/djmitche/buildbot into nine