Bump python-swiftclient requirement to >=1.5

python-swiftclient 1.5.0 has new functionality (the ability to disable
SSL layer compression) which Glance would like to use.

Change-Id: Ibbe1b53d81f8ee8b907d5270742449644505c3ad
Merge "Bump python-swiftclient requirement to >=1.5"