[Runtimes] Support configuring auto mount type

[theSaarco]

Support the requirements in https://jira.iguazeng.com/browse/ML-867 - perform auto-mount on runtimes, saving the user the need to use func.apply(auto_mount()) for every function deployed remotely (and actually not using auto_mount as it will default to fuse mount on every function, which is usually not needed).
Two configuration parameters were added:

• storage.auto_mount_type - based on the values in the AutoMountType enum. By default this is auto, and will also be the case if the parameter is empty. Possible values are:
  • none - self explanatory
  • v3io_credentials - add v3io credentials to the deployed runtime (applies v3io_cred)
  • v3io_fuse - mount v3io fuse driver (applies mount_v3io)
  • pvc - configure PVC parameters (applies mount_pvc)
  • auto - for Iguazio system will use v3io_credentials, else will be mount_pvc if pvc is configured or none otherwise
• storage.auto_mount_params - a string containing kwargs to be passed to the mount function. The exact kwargs to use depend on the type of the auto mount in use and the function it invokes. The format of the string is pairs of values separated by commas: "param1=value1,param2=value2"

Auto mount is activated on func.run, except for Nuclio (RemoteRuntime) where it's applied in func.deploy. Auto mount is always performed on the client side. It is also applied for workflows, immediately after running the user's init_functions code (if exists).
Auto mount is disabled if someone called apply on the runtime with any other modifier. Of course, this also means that if auto-mount was already applied, it won't be applied again.

There are 2 other work-items which are related and are not part of this PR:

1. Documentation fixes - given that current docs always apply auto-mount to functions prior to their deployment, this will have no effect on the existing docs/demos. However, the docs need to reflect this change.
2. Properly configure these parameters when deploying MLRun kit - ideally the auto-mount parameters automatically point at the NFS mount. Possibly modify the Helm chart so that the user can control that as part of the values passed to it.

[Hedingber]

Also reminding ourselves to talk about the mount_pvc thing in the auto mode

[Hedingber]

I feel like it should be else pvc if X is configured (need to think what is that X) else None

[Hedingber]

Looks good just please update the comment near the auto_mount_type in config.py and I'll merge