Change directory which downloading snapshot to

Change the downloading directory, which named by build date.

Change-Id: I4f04d7090a79f23dd7db0842bf540184ed137b37
Signed-off-by: gaoxuesx <xuesongx.gao@intel.com>
Fix bugs of UserCache.

There is bug in user _eq function. Have to check fields existence
before comparing them.

Ignore users without email, otherwise it's hard to make a unique
value for username field.

Add several tests for above cases.

Change-Id: I7cc07192e57f723607c4f66c45785d9e4a6fa1f9
Change scm blocks parsing rules.

Now scm blocks are separated by empty lines. By following this
definition, we can put domain string and git tree string together,
which makes parse_blocks() only need one argument. And makes
testing functions more readable.

Change-Id: I91d595556069295a395082d78be2cba303d98c9f
Install latest python-requests: 2.3.0

Change-Id: I0175a2d21bdcdbf0a46985514dfeb0aa7993901f