checkbox-touch: move app_blob creation outside start_session

This patch moves creation of app_blob outside start_session so it may be
reused by other functions needing to packit befor checkpoint()-ing
plainbox:session:state: add 'bootstrapping' flag

This patch introduces 'bootstrapping' flagged to that might be used in
SessionMetaData. This flag informs about the session is established.
If this flag is the only one present, then it implies that tests have not
yet been started.
checkbox-touch: add resume session functionality

This patch enables resume-session functionality in checkbox-touch.
After session is established session's metadata flag 'bootstrapping' is set
instead of 'incomplete', which is now set when tests are selected.

Fixes: https://bugs.launchpad.net/checkbox-touch/+bug/1375589
Signed-off-by: Maciej Kisielewski <maciej.kisielewski@canonical.com>
checkbox-touch: outlined session storage creation
checkbox-touch: fix PEP8 issues
"automatic merge by tarmac [r=zkrynicki][bug=1375589][author=kissiel]"