Fixes #264

Add API support for share agnostic nfs-export management.
Add stub code for frontend storage->nfs view
Stub code for issue #264
Merge branch 'issue#264' of https://github.com/schakrava/rockstor-core into schakrava-issue#264
Fix template errors.
NFS exports at top level

Add nfs export creation form
NFS exports

Display nfs export groups
NFS exports

Create edit view and template
Update nfs export model for top level nfs operations. Create a new model for
export groups that represent one or more exports for same host and option combination.
refactor nfs helper code to efficiently update exports
Implement nfs export group API
Update snapshot view for nfs export groups
Update bootstrap command for nfs export groups
Display load average

Add before function in router to check if loadavg is displayed
Set timer to fetch uptime/loadavg at an interval of 10 seconds
Display loadavg/uptime next to the appliance name
Bugfix: call the right method from nfs helpers
NFS exports

Add delete method and action icon in nfs exports table
bugfix: fix the regex in nfs-export api url
Update DELETE of nfs-export so it also cleans up unnecessary mounts
NFS exports

Re-render exports page on deletion
Make exporting/unexporting transparent from both storage level and share level
nfs interfaces.