Feat!: Re-factor lf-activate-venv() to re-use venv

Feat!: Re-factor lf-activate-venv() to re-use venv

Add new CLI option to set venv file.

Example:

lf-activate-venv --venv-file /tmp/.robot_venv \
	robotframework

Modify lf-activate-venv() to allow creation of a venv file and re-use
the venv to improve job performance. When a dependency is already
installed, pip skips the package therefore reduces the time it takes
to create venv in every script.

Precedence for venv file.
  a. Re-use an existing venv file if one exists.
       1. Use venv file path from --venv-file
       2. Use default venv file path "/tmp/.os_lf_venv"
  b. Create new venv when 1. and 2. is absent

Note: The default file "/tmp/.os_lf_venv" is created by a pre-build
script (../shell/python-tools-install.sh).

In the situation where a fresh venv is required remove
"/tmp/.os_lf_venv" before calling lf-activate-venv().

Update all the required scripts that call lf-activate-venv().

Issue-ID: RELENG-4403
Change-Id: I559f759a8dba7eca0a62f8b73a360dc627699ed2
Signed-off-by: Anil Belur <abelur@linuxfoundation.org>