avocado.utils.process: add support for environment variables.

The main SubProcess class now supports an argument (env) to receive
extra environment variables to the subprocess execution.

Note that it's extra in the sense that it includes the
current environment variables, so the previous environment is
not destroyed.

The utility functions now accepts this extra environment variables.
It's more robust to pass variables as a dictionary than
using the shell feature of shell variables.

Signed-off-by: Rudá Moura <rmoura@redhat.com>
avocado.utils.build: Don't use shell feature to pass environment variables.

Don't use the shell like feature to pass environment variables
to the make command, instead of make use of the new feature
in module avocado.utils.process to pass environment variables.

Plus make use of argument make to define the make command name.

Signed-off-by: Rudá Moura <rmoura@redhat.com>
examples.tests: Don't use shell feature to pass environment variables.

Don't use the shell like feature to pass environment variables
to the make command, instead of make use of the new feature
in module avocado.utils.process to pass environment variables.

Signed-off-by: Rudá Moura <rmoura@redhat.com>
Merge pull request #253 from ruda/subprocess_with_env

Include support to add extra environment variables in module `avocado.utils.process`.