Allow passing environment variables to execute()

Some of the projects might need to pass environment variables
to the processes they launch. For instance, if something needs
to rely on the process' output it should be able to pass LANG="C"
in order to set the correct localization.

This patch adds a env_variables parameter to processutils.execute()
which allows to pass news variables or replace the value of the existing
ones for the process.

Closes-bug: #1304284
Change-Id: If801e11391bf7cf66518533443c3b29af53653e5
Fixed several typos

Change-Id: I6ad8231ace0d123f9700c649926da3eb85eb4716
Update update.py for oslo.i18n

oslo.i18n was getting replaced with `base`.i18n (where base is the
target project).

Change-Id: If4281335fa5596966873f8728de77222c5726ad5
Merge "Fixed several typos"
Merge "Update update.py for oslo.i18n"
Update the status of gettextutils to graduating

bp graduate-oslo-i18n

Change-Id: Ic82b8cc59954b13c691ee298152aff8bc97e1346
Merge "Allow passing environment variables to execute()"
Merge "Update the status of gettextutils to graduating"