Big Switch: fix capabilities retrieval code

References the raw REST response in the capabilities
parsing code so json.loads doesn't get an object
that may already be decoded and fail.

Closes-Bug: #1326173
Change-Id: Ia3179b7499f35a6ab2e9ce1631ab15ed27d64647
add engine parameter for offline migrations

Offline migration required config file containing connection string.
In that case only engine from URL was used. With this patch engine can be
passed from command line (or config file) along with plugins which sql
script will be generated accordingly.

DocImpact

Change-Id: Ib667a71960b833fd981f97fe7d6b1856084ef5c8
Closes-Bug: #1304326
Merge "Big Switch: fix capabilities retrieval code"
Merge "add engine parameter for offline migrations"