Online volume size extending support #1126

Is this a BUG REPORT or FEATURE REQUEST?:

Uncomment only one, leave it on its own line:

/kind feature

What happened:
I am writing CSI ExtendVolume feature, but hotpot only supports offline volume size extending. it will fail once volume is attached to a host.

What you expected to happen:

hotpot should allow extending in-use volumes.

How to reproduce it (as minimally and precisely as possible):

1. Create a host
2. Create a volume
3. Attach a volume to a host
4. Extend volume size

Anything else we need to know?:

Environment:

• Hotpot(release/branch) version: v0.7.0
• OS (e.g. from /etc/os-release): ubuntu 16.04
• Kernel (e.g. uname -a):
• Install tools:opensds-installer
• Others: