tox_install: Don't leave IFS set

When running "tox -e docs" in my environment, git clone fails with
"Too many arguments."  It seems ":" in "https://" is affected by IFS.

1. In tox_install.sh, IFS is set to ":". It affects the current
  execution environment and can have undesirable side effects later.
  It's safer to restore it.  (This commit fixes this.)

2. Otoh, the particular symptom above seems like a bug in a version
  of bash I happen to have.

  bash-3.2$ bash --version
  GNU bash, version 3.2.57(1)-release (x86_64-apple-darwin14)
  Copyright (C) 2007 Free Software Foundation, Inc.
  bash-3.2$ IFS=:
  bash-3.2$ echo a:b
  a:b
  bash-3.2$ echo a:b$x
  a b
  bash-3.2$

Closes-Bug: #1708147
Change-Id: I897367bf873c02bbf16162c9b6e2fb5b63ad3e3a

tools/tox_install.sh

@@ -71,6 +71,7 @@ while [ $# -gt 0 ] ; do
     os:*)
         declare -a pkg_spec
         IFS=: pkg_spec=($1)
+        unset IFS
         install_from_git "${pkg_spec[1]}" "${pkg_spec[2]}"
     ;;
     # Otherwise just pass the other deps through to the constrained pip install