Support for stored snapshot -- Fix #132

[eeishaan]

Fix #132
Added support for stored snapshot.

Usage:

1. First one needs to create a snapshot
   jsnapy --snap STORED -f config.yml
2. To run snapcheck on this stored snapshot from command line, use --local parameter
   jsnapy --snapcheck STORED -f config.yml --local
3. One can also specify the stored snapshots' name in the configuration file as follows:

hosts:
  - device: 10.216.193.114
    username : abc
    passwd: xyz
tests:
  - delta.yml
local:
  - STORED
  - STORED_42
  - STORED_314
sqlite:
  - store_in_sqlite: no
    check_from_sqlite: no
    database_name: jbb.db
    compare: 0,1

Things to Note:

1. The devices and the various commands passed in the configuration file passed should match with the ones passed to the tool while capturing the snapshot. One can modify the various test operators.
2. If a user wants to run local snapcheck on his own xml file, he need to modify the name of that xml to the format given below, place it in the snapshots directory and modify the configuration file accordingly.
   hostname_snapname_command.xml
   or
   hostname_snapname_rpc.xml

For module version:

1. One needs to pass additional parameter local=True while calling snapcheck() to use local snapcheck. The snapname passed is considered as the local snapshot to run snapcheck on, just like the command line. This parameter is not required if the configuration file contains the list of snapshot names as above.
2. The configuration file support for local as expected is also baked in.
3. Return Types:
4. If one specifies the local keyword in the configuration file, then the snapcheck() function returns a list of dictionary, where the dictionary is keyed with the various snapnames passed and the value is an Operator object which contains the test details.
5. In every other case, a list of Operator object which contains the test details is returned