Add user-specified download path for lftools sign

Add user-specified download path for lftools sign

Add a user-specified download location for lftools sign
deploy-nexus command. The default download location
is "/tmp/gpg-signatures.*" where '*' is random generated
suffix added to a directory name returned by mkdtemp()

Projects like ODL require more than 5 GiB free space
which might not be available in the returned path
therefore we should allow the user to use an alternative
download path.

ex:

lftools sign deploy-nexus -d <path> <URL> <repo> <profile-id>

JIRA: RELENG-928
Change-Id: I960e6c2dd5b5311ec0551834c6b96a297a2eb8bd
Signed-off-by: Anil Belur <abelur@linuxfoundation.org>